Physical and Biological Treatment Technologies of Slaughterhouse Wastewater: a Review

sustainability

Review Physical and Biological Treatment Technologies of Slaughterhouse Wastewater: A Review

Mohammed Ali Musa 1,2 and Syazwani Idrus 1,*

1 Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia; [email protected] 2 Department of Civil and Water Resources Engineering, University of Maiduguri, Maiduguri P.M.B. 1069, Borno State, Nigeria * Correspondence: [email protected]; Tel.: +60-13-692-2301

Abstract: Physical and biological treatment technology are considered a highly feasible and economic way to treat slaughterhouse wastewater. To achieve the desired effluent quality for disposal or reuse, various technological options were reviewed. However, most practical operations are accompanied by several advantages and disadvantages. Nevertheless, due to the presence of biodegradable organic matter in slaughterhouse waste, anaerobic digestion technology is commonly applied for economic gain. In this paper, the common technologies used for slaughterhouse wastewater treatment and their suitability were reviewed. The advantages and disadvantages of the different processes were evaluated. Physical treatments (dissolved air floatation (DAF), coagulation–flocculation and sedimentation, electrocoagulation process and membrane technology) were found to be more effective but required a large space to operate and intensive capital investment. However, some biological treatments such as anaerobic, facultative lagoons, activated sludge process and trickling filters were also effective but required longer start-up periods. This review further explores the various strategies being used in the treatment of other wastewater for the production of valuable by-products through Citation: Musa, M.A.; Idrus, S. anaerobic digestion. Physical and Biological Treatment Technologies of Slaughterhouse Keywords: wastewater; slaughterhouse wastewater; physical treatment; biological treatment Wastewater: A Review. Sustainability 2021, 13, 4656. https://doi.org/ 10.3390/su13094656 1. Introduction Academic Editor: Haoran Duan The effective treatment of high-strength industrial wastewater has increased over

Received: 13 February 2021 time due to the effects related to environmental pollution. The discharge of untreated Accepted: 25 March 2021 slaughterhouse wastewater (SWW) constitutes a severe threat to public health and the Published: 22 April 2021 environment [1]. Although rivers have a natural cleansing capacity, the frequent release of such efﬂuent without it being adequately treated ﬁrst might overburden the receiving water

Publisher’s Note: MDPI stays neutral body. Today, the management of wastewater needs to incorporate both waste minimization with regard to jurisdictional claims in and resource recovery [2]. Although fresh water consumption by different slaughterhouse published maps and institutional afﬁl- industry varies in magnitude and concentration, it is usually preferable to minimize iations. wastewater generation at its source. The wastewater generated from a slaughterhouse consists of organic by-products which are considered industrial organic wastes, which are challenging to treat due to their high protein and lipid contents. The main organic streams that portrayed SWW as recalcitrant in nature include the blood, paunches from the

Copyright: © 2021 by the authors. removal of the rumen and the intestinal content, the intestinal residues from the evisceration Licensee MDPI, Basel, Switzerland. process [3], fats from the meat trim step as well as the head and the limbs (mostly bones). This article is an open access article Conventionally, SWW treatment methods are similar to the current technologies used in distributed under the terms and municipal wastewater treatment, which include physicochemical and biological treatments conditions of the Creative Commons where each method has its advantages and disadvantages. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Sustainability 2021, 13, 4656. https://doi.org/10.3390/su13094656 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, x FOR PEER REVIEW 2 of 19 Sustainability 2021, 13, 4656 2 of 20

1.1. Physicochemical Treatment 1.1. Physicochemical Treatment Physicochemical treatment methods usually involve solid separation from the fluid. Physicochemical treatment methods usually involve solid separation from the fluid. It is recommended that the effluent be sent for primary or secondary treatment after the It is recommended that the effluent be sent for primary or secondary treatment after preliminary treatment depending on the intensity of the SWW [4]. Dissolved air floatation the preliminary treatment depending on the intensity of the SWW [4]. Dissolved air floatation(DAF), coagulation (DAF), coagulation–flocculation–flocculation and sedimentation, and sedimentation, electrocoagulation electrocoagulation process and process mem- andbrane membrane technology technology are usually are employed usually employed as primary as treatment primary treatment technologies technologies for the treat- for thement treatment of SWW of[5,6]. SWW In the [5, 6analysis]. In the of analysis samples of, the samples, standard the methods standard for methods the examination for the examinationof water and ofwastewater water and of wastewater the American of thePublic American Health PublicAssociation Health [7] Association are commonly [7] areapplied, commonly to achieve applied, chemical to achieve oxygen chemical demand oxygen (COD) demand, biochemical (COD), oxygen biochemical demand oxygen (BOD), demandtotal suspended (BOD), totalsolid suspended(TSS), volatile solid suspended (TSS), volatile solids suspended(VSS), ammonia solids nitrogen (VSS), ammonia (NH3-N), fats, oil and grease (FOG), colour and turbidity removals. nitrogen (NH3-N), fats, oil and grease (FOG), colour and turbidity removals.

1.1.1.1.1.1. DissolvedDissolved AirAir FloatationFloatation (DAF)(DAF) DissolvedDissolved air air ﬂoatation floatation is is simply simply the the introduction introduction of of air air from from the the bottom bottom ofthe of the system sys- fortem liquid–solid for liquid–solid separation, separation as shown, as shown in Figure in Figure1. During 1. During operation, operation, the FOG the light FOG solid light materialssolid materials are transported are transported to the surface,to the surface, creating creating a sludge a blanket.sludge blanket. Thus, the Thus, scum the formed scum isformed continuously is continuously removed removed by scrapping. by scrapping.

FigureFigure 1.1. SchematicSchematic diagramdiagram ofof thethe dissolveddissolved airair ﬂoatationfloatation clariﬁerclarifier unitunit [8[8]]..

PolymersPolymers andand otherother flocculantsflocculants areare usuallyusually appliedapplied toto enhance the efficiencyefficiency of DAF. InIn treatingtreating SWW, SWW, ferric ferric chloride chloride and and aluminium aluminium sulphate sulphate are are usually usually added added to facilitate to facilitate the aggregationthe aggregation and and precipitation precipitation of protein of protein in addition in addition to fat to and fat greaseand grease floatation. floatation. Moreover, More- 30over, to 90%30 to COD, 90% COD, as well as aswell 70 as to 70 80% to BOD80% BOD removal removal efficiency efficiency can becan achieved be achieved using using the DAFthe DAF process. process. Furthermore, Furthermore, Mittal. Mittal. [9] [9 and] and De De Nardi Nardi et et al. al. [5 [5]] have have shown shown that that the the DAF DAF systemsystem isis capablecapable ofof achievingachieving moderatemoderate toto highhigh nutrientnutrient removal.removal. FloatationFloatation cancan alsoalso bebe usedused asas anan alternativealternative methodmethod ofof handlinghandling pulppulp andand paperpaper millmill effluenteffluent inin additionaddition toto firmfirm settling.settling. TheseThese devicesdevices injectinject aa pressurizedpressurized flow flow of ofair-saturated air-saturated water water at at the the base base of of aa deepdeep chestchest thatthat holdsholds thethe paperpaper millmill processprocess steam.steam. TheThe injectedinjected water s released released into into the the chest, chest, and and tiny tiny air air bubbles bubbles come come out out of ofthe the so- solutionlution and and start start to to rise. rise. The The rising rising bubbles bubbles tend tend to to carry any otherother fairlyfairly solidsolid bindingbinding particlesparticles and and can can easily easily be be skimmed skimmed from from the the water’s water’s surface. surface. DAF’s DAF’s mainmain drawback, drawback, however,however, isis commonlycommonly associatedassociated withwith relativelyrelatively frequentfrequent systemsystem failurefailure andand inefficientinefficient TSS separation [10]. Therefore, an alternative treatment system like the upflow anaerobic TSS separation [10]. Therefore, an alternative treatment system like the upflow anaerobic sludge blanket reactor is required, due to its lesser energy demand, smaller ecological foot sludge blanket reactor is required, due to its lesser energy demand, smaller ecological foot print production as well as its overall operation and maintenance cost. print production as well as its overall operation and maintenance cost. 1.1.2. Coagulation–Flocculation and Sedimentation 1.1.2. Coagulation–Flocculation and Sedimentation The addition of coagulant into a reactor vessel containing SWW promotes the formation ofThe large addition colloidal of coagulant particles, whichinto a reactor are called vessel flocs. containing The colloidal SWW particles promotes produced the for- inmation SWW, of however, large colloidal are negatively particles, charged, which are making called themflocs. stableThe colloidal and aggregation particles produced resistant. Coagulantsin SWW, however, with positively are negatively charged charged, ions are making therefore them added stableinto and theaggregation vessel for resist properant. flocCoagulants formation with in positively order to destabilize charged ions the are colloidal therefore particles added to into form the flocs vessel and for ease proper the Sustainability 2021, 13, x FOR PEER REVIEW 3 of 19

Sustainability 2021, 13, 4656 3 of 20

floc formation in order to destabilize the colloidal particles to form flocs and ease the sedimentation process. Chemicals such as ferric chloride, ferric sulphate, aluminium sul- sedimentationphate, aluminium process. chlorohydrate, Chemicals and such poly as ferric-aluminium chloride, chloride ferric sulphate,were used aluminium as coagulants sulphate,for the aluminiumSWW treatment. chlorohydrate, The use andof poly poly-aluminium-aluminium chloride chloride as were reagent used showed as coagulants a total forphosphorus the SWW (TP treatment.), total nitrogen The use (TN of poly-aluminium), and COD removals chloride efficiencies as reagent of up showed to 99.9, a total88.8, phosphorusand 75.0%, respectively. (TP), total nitrogen On the (TN), other and hand, COD the removals sludge volume efﬁciencies can be of upreduced to 99.9%, by 41.6 88.8%, % andusing 75.0%, inorganic respectively. coagulants On the[9,11]. other Figure hand, 2 the demonstrates sludge volume coagulation can be reduced–flocculation by 41.6% and using inorganic coagulants [9,11]. Figure2 demonstrates coagulation–ﬂocculation and sedimentation processes. sedimentation processes.

Figure 2. Schematic representation of coagulation–flocculation processes [12]. Figure 2. Schematic representation of coagulation–flocculation processes [12]. Satyanarayan et al. [13] have reported the use of anionic polyelectrolyte, ferrous sulphate,Satyanarayan lime, and et alum al. [13] as coagulants have reported in the the treatment use of anionic of SWW. polyelectrolyte, The results revealed ferrous BOD, sul- COD,phate, andlime, TSS and removal alum as efficiencies coagulants of in 38.9, the 36.1,treatment and 41.9% of SWW. using The only results lime revealed as a coagulant. BOD, ACOD, significant and TSS improvement removal efficiencies in COD removalof 38.9, 36.1, up to and 56.8% 41.9% was using realized only in lime the combinationas a coagulant. of ferrousA significant sulphate improv withement lime. Likewise,in COD removal an increase up to in 56.8 COD% was removal realized to 42.6% in the was combination recorded inof theferrous combination sulphate ofwith alum lime. and Likewise, lime. Tariq an increase et al. [14 in] investigated COD removal the to use 42.6 of% alum was andrec- limeorded individually in the combination in the treatment of alum and of SWW. lime. ItTariq was revealedet al. [14] thatinvestigate with thed increasingthe use of alum dose ofand alum, lime theindividually COD removal in the reached treatment a maximum of SWW. ofIt was 92% revealed along with that high with sludge the increasing volume, anddose this of alum, rendered the COD the process removal inefficient. reached a Conversely,maximum of 74% 92% COD along reduction with high was sludge realized vol- withume, anand increasing this render dosageed the of process lime as inefficient. a single coagulant. Conversely, Comparatively, 74% COD reduction the sludge was volume real- generatedized with an using increasing lime was dosage quite lowof lime compared as a single to that coagulant. of alum. Comparatively, However, the combination the sludge ofvolume the two generated coagulants using revealed lime was a maximum quite low CODcompared removal to that of 85% of alum. with However, a small quantity the com- of sludgebination volume. of the two coagulants revealed a maximum COD removal of 85% with a small quantityDifferent of sludge contaminants volume. can be removed from the wastewater through coagulation/ flocculationDifferent which contaminants would otherwise can be be removed difficult withoutfrom the the wastewater application ofthrough these chemicals. coagula- Limitedtion/flocculation investment which is required would forotherwise these tanks be anddifficult dosing without units. Nevertheless,the application the of operat- these ingchemicals. costs are Limited a major investment disadvantage is required of this strategy. for these In tanks some and situations, dosing significantunits. Nevertheless, amounts ofthe coagulant operating andcosts flocculent are a major are disadvantage needed to achieve of this the strategy. required In some level ofsituations, flocculation. signifi- A certaincant amounts amount of ofcoagulant physico–chemical and flocculent sludge are isneeded also produced, to achieve which the required is usually level handled of floc- externally.culation. A Thesecertain costs amount may of escalate, physico– especiallychemical sludge with large is also volumes produced, of wastewater. which is usually The correcthandled dosage externally. of chemicals These is also costs very may important escalate, for especially the proper functioning with large of volumes the process. of Therefore,wastewater. this The is notcorrect simple dosage for sewage of chemicals with widely is also varyingvery important composition. for the proper functioning of the process. Therefore, this is not simple for sewage with widely varying com- 1.1.3. Electrocoagulation (EC) Process position. Electrocoagulation requires the production of in situ coagulants by electrically dissolv- ing1.1.3. aluminium Electrocoagulation or iron from (EC) aluminium Process or iron electrodes, respectively. Figure3 shows the schematic diagram of electrocoagulation processes. Metal ions are produced at the anode and hydrogenElectrocoagulation gas is emitted requires from the the production cathode. of Hydrogen in situ coagulants gas would by also electrically help lift dis- the flocculatedsolving aluminium particles outor ironof the from air. The alumin electrodesium or can iron be setelectrodes, in a monopoly respectively. or bipolar Figure mode. 3 Theshows products the schematic may be madediagram of aluminiumof electrocoagulation or iron in the processes. form of platesMetal orions the are form produced of scraps, at suchthe anode as steel and turning hydrogen and gas milling. is emitted The ECfrom process the cathode. is an advanced Hydrogen treatment gas would technology also help recentlylift the flocculated applied to particles the treatment out of of the SWW. air. AccordingThe electrodes to Emamjomeh can be set in and a monopoly Sivakumar or [ 15bi-] andpolar Bayar mode. et The al. [16 products], the system may be is capablemade of of aluminium removing pathogens,or iron in the organics, form of nutrients, plates or and the even heavy metals from SWW by introducing an electric current without the addition of any chemical. Electrodes such as Al, Fe, Pt, SnO2, and TiO2 are commonly utilized for Sustainability 2021, 13, x FOR PEER REVIEW 4 of 19

form of scraps, such as steel turning and milling. The EC process is an advanced treatment technology recently applied to the treatment of SWW. According to Emamjomeh and Si- vakumar [15] and Bayar et al. [16], the system is capable of removing pathogens, organics, nutrients, and even heavy metals from SWW by introducing an electric current without

Sustainability 2021, 13, 4656 the addition of any chemical. Electrodes such as Al, Fe, Pt, SnO2, and TiO2 are commonly4 of 20 utilized for the EC process, however, Al and Fe are the most widely applied. In the EC process, M3+ ions are usually generated on-site with the help of sacrificial anodes. More- over, studies have shown that these sacrificial electrodes might interact with hydrogen the EC process, however, Al and Fe are the most widely applied. In the EC process, M3+ ionsions in an are acidic usually medium generated or with on-site an with OH the- ion help in an of sacrificialalkaline medium anodes. Moreover,[17–20]. For studies instance, the haveresearch shown of thatKobya these etsacrificial al. [18] into electrodes the EC might process interact treating with hydrogenSWW demonstrate ions in an acidicd that Al, as anmedium electrode or with material an OH- in ionthe inEC an process alkaline, was medium responsible [17–20]. Forfor instance,removing the up research to 93% of COD, whereasKobya Fe et as al. an [18 electrode] into the EC material process was treating able SWWto achieve demonstrated a maximum that Al,of 98% as an oil electrode and grease removalmaterial efficiency. in the EC During process, wasthis responsibleprocess, the for influential removing up parameters to 93% COD, that whereas lead Feto asthe an high COD,electrode oil, and material grease was removal able to include achieved a the maximum pH, operating of 98% oil time, and grease electrode removal material, efficiency. and the currentDuring density. this process, the influential parameters that lead to the high COD, oil, and grease removal included the pH, operating time, electrode material, and the current density.

FigureFigure 3. Schematic 3. Schematic representation representation of of the the electrocoagulationelectrocoagulation processes processes [21 ].[21].

An evaluation of the chemical reactions that occur in the process of electrocoagulation An evaluation of the chemical reactions that occur in the process of electrocoagula- reveals that the main electrode reactions (aluminium electrodes) are: tion reveals that the main electrode reactions (aluminium electrodes) are: Anode: Al → Al3+ (aq) + 3e (1) Anode: Al → Al3+ (aq) + 3e (1) − Cathode: 3H2O + 3e → 3/2H2 + 3O − Cathode: 3H2O + 3e →3/2H2 + 3O The cathode may also be chemically attacked by HO− ions generated during H2 The cathode may also be chemically attacked by HO− ions generated during H2 evo- evolution at high pH [22]: lution at high pH [22]: − − 2Al + 6H2O + 2HO → 2Al (HO)4 + 3H2 − − 2Al + 6H2O + 2HO → 2Al (HO)4 + 3H2 Al3+ (aq) and OH− ions generated by electrode reactions (1) react to form various 3+ − 2+ + 4+ − monomericAl (aq) and species OH such ions as Al generated (OH) , Al by (HO) electrode2 ,Al2 ( HO reactions)2 and (1) Al react(HO) 4 to, formand poly- various 3+ 2+ 4++ 4+ − 7+ monomericmeric species species such such as Al as6 ( HOAl )(OH)15 ,A,l7 Al(HO (HO)17)2 ,, AlA8l2( (HOHO))202 and, and Al Al 13(HOO4)(4HO, and)24 poly-, ( ) 5+ 3+ 4+ 4+ 7+ mericAl13 speciesHO 34 such, which as Al finally6 (HO transform)15 , Al7 into (HO Al)17 (OH), 3Alaccording8 (HO)20 to complex, and Al precipitation13 O4 (HO)24 , kinetics [523+]. Al13 (HO)34 , which finally transform into Al (OH)3 according to complex precipitation kinetics1.1.4. [23] Membrane. Technology Membrane technology is becoming more popular in the treatment of water and 1.1.4.wastewater Membrane due Technology to regulatory issues towards meeting the stringent water quality requirements.Membrane Microfiltration technology (MF), is ultrafiltration becoming more (UF), nanofiltration popular in the (NF) treatment and reverse of osmosis water and wastewater(OS) are the due common to regulatory membrane issues technologies towards usedmeeting for water the stringent purification. water Figure quality4 depicts requirements.the differentMicrofiltration membrane (MF), sizes ultrafiltration for the treatment (UF), of water nanofiltration and wastewater. (NF) Dependingand reverse on osmosis the (OS)pore are size,the common membranes membrane can remove technologies colloids, particles, used andfor water macromolecules. purification. This Figure technology 4 depicts is increasingly applied in the treatment of SWW to remove organic matter and bacteria [24]. the different membrane sizes for the treatment of water and wastewater. Depending on The performance of RO in the treatment of secondary effluent of SWW (activated sludge the aspore pre-treatment) size, membranes was reported can remove by Bohdziewicz colloids, andparticles, Sroka and [25]. macro The resultmolecules. of parameters This tech- nologylike BOD,is increasingly COD, TN, andapplied TP were in foundthe treatment as 50.0, 85.8, of SWW 90.0, and to 97.5%,remove respectively. organic matter It was and concluded that RO was a suitable technique for the post-treatment of SWW effluent. The study of Yordanov [26] on the performance of the UF membrane treating SWW showed Sustainability 2021, 13, x FOR PEER REVIEW 5 of 19

bacteria [24]. The performance of RO in the treatment of secondary effluent of SWW (activated sludge as pre-treatment) was reported by Bohdziewicz and Sroka [25]. The result Sustainability 2021, 13, 4656 of parameters like BOD, COD, TN, and TP were found as 50.0, 85.8, 90.0, and5 of 20 97.5%, respectively. It was concluded that RO was a suitable technique for the post-treatment of SWW effluent. The study of Yordanov [26] on the performance of the UF membrane treat- BODing and SWW COD showed removal BOD efﬁciencies and COD of around removal 97.8–97.89 effici toencies 94.52–94.74%, of around whereas 97.8 TSS–97.89 and to 94.52– FOG94.74%, removal whereas were 98TSS and and 99%, FOG respectively. removal were 98 and 99%, respectively.

FigureFigure 4. 4.Schematic Schematic representation representation of pressure-driven of pressure-driven membrane membrane ﬁltration filtration [27]. [27].

TheThe investigation investigation of Gürelof Gürel and and Büyükgüngör Büyükgüngör [28] indicated [28] indicate that ad membrane that a membrane biore- biore- actorsactor coulds could significantly significantly remove remove nutrients nutrients and other and organicsother organics from SWW. from A SWW. pilot-scale A pilot-scale experimentexperiment of anaerobic of anaerobic submerged submerged bioreactor bioreactor membrane (SAMBR) membrane treating (SAMBR) high-strength treating high- wastewater (raw tannery wastewater) achieved a higher COD removal efficiency of up strength wastewater (raw tannery wastewater) achieved a higher COD removal efficiency to 90% at 6 g/L·day organic loading rate (OLR) and biogas production (0.160 L g COD removed)of up to [9029].% Theat 6 process g/L·day worked organic efficiently loading but rate was (OLR) strongly and characterized biogas production by a high (0.160 L g hydraulicCOD removed) retention [29]. time The (HRT) process of 40 h, worked and as suchefficiently high energy but was was strongly spent, although characterized the by a permeatehigh hydraul flux remainedic retention at (6.8 time LMH) (HRT) well of below 40 h the, and critical as such flux high (17.5 LMH)energy as was established spent, although inthe the permeate earlier work flux of remained Hu and Stuckey at (6.8 LMH) [30]. Most well recently, below the the critical filtration flux performance (17.5 LMH) of as estab- anlished anaerobic in the membrane earlier work bioreactor of Hu (AnMBR)and Stuckey treating [30]. high Most strength recently, lipid-rich the filtration wastewater performance andof corn-to-ethanol an anaerobic thin membrane stillage was bioreactor conducted by (AnMBR) Dereli et al. treating [31]. The highreactors strength delivered lipid-rich awastewater high COD removal and corn efficiency-to-ethanol of up thin to 99% stillage under was stable conducted operating by conditions Dereli et al. with [31]. an The reac- average OLR of 8.3, 7.8 and 6.1 kg COD/m3·day. However, the permeate turned out to tors delivered a high COD removal efficiency of up to 99% under stable operating condi- be inferior in quality with an increased solid retention time (SRT). Generally, membrane 3 lifetimetions with remains an average the main OLR concern of 8.3, of investors 7.8 and in6.1 the kg water COD/m treatment·day. andHowever, wastewater the permeate industries.turned out The to efficiencybe inferior of in reversing quality fouling with an on increased the membrane solid surface retention is being time exploited (SRT). Generally, bymembrane physical, chemical, lifetime and remains biological the methods. main concern Although of thereinvestors were enoughin the water physical treatment and and chemicalwastewater methods, industries. the disadvantages The efficiency are of enormous. reversing During fouling aeration, on the membrane much energy surface is is be- expended,ing exploited and sometimesby physical, chemicals chemical, are and used biological for membrane methods. cleaning, Although and this there activity were enough doesphysical not benefit and chemical the players methods, in this field the in disadvantages terms of cost and are environment. enormous. During aeration, much energy is expended, and sometimes chemicals are used for membrane cleaning, and this 1.1.5. Summary of Physicochemical Treatment Methods activity does not benefit the players in this field in terms of cost and environment. Table1 summarizes the advantages and disadvantages of the different physicochemical treatment methods of slaughterhouse wastewater. 1.1.5. Summary of Physicochemical Treatment Methods Table 1. AdvantagesTable 1 summarizes and disadvantages the advantages of physicochemical and disadvantages methods. of the different physicochemical treatment methods of slaughterhouse wastewater. Methods Advantage Disadvantage

High energy demand due to aeration. It can achieve 30–90% COD and 70–80% Chemical addition which renders the sludge BOD removal efficiencies. unsuitable as fertilizer. Dissolved air Moderate to high nutrient removal. Inefficient total suspended solid separation. floatation Tends to carry fairly solid binding particles Lacks energy recovery facilities. and can easily be skimmed from the water’s Frequent system failure. surface. High cost of operation and maintenance. Sustainability 2021, 13, 4656 6 of 20

Table 1. Cont.

Methods Advantage Disadvantage

Huge quantity of chemical is applied.

Promotes large colloid formation which can Large volume of sludge is generated causing an Coagulation– easily sediment. additional cost of treatment.

flocculation and TP, TN, and COD removals efficiencies of up Difficult to handle or reuse. sedimentation to 99.9%, 88.8%, and 75.0% can be achieved. Landfill disposal or incineration is usually the only option available to handle the sludge.

Lacks energy generating facilities.

The system is capable of removing High energy demand and not cost effective. pathogens, organics and other nutrients by Electrocoagulation Lack energy generation facilities especially in the introducing electric current. (EC) process treatment of organic wastewater to high energy High COD and FOG removal efﬁciency potentials. (>90%).

Characterized by frequent fouling. Depending on the type of membrane, the During aeration, much energy is expended, and technology is capable of achieving sometimes chemicals are used for membrane Membrane 97.8–97.89% and 94.52–94.74% BOD and cleaning. technology COD removal efﬁciencies in the treatment of High energy input and zero energy output slaughterhouse wastewater. especially in the treatment of slaughterhouse wastewater.

2. Biological Treatment In the treatment of SWW, biological treatment is applied as a secondary treatment to reduce the concentration of BOD and other soluble compounds after primary treatment [32]. Depending on the characteristics of SWW, the biological process is applied when aerobic and anaerobic digestion are operating individually or as combined systems with packing material [33]. Unlike the physicochemical process, the biological treatment process employs the use of microorganisms to remove organics from SWW effluent. Mittal [9] demonstrated that the biological method that properly applies the aerobic or anaerobic process could remove about 90% BOD from SWW effluent. There exist different biological systems, which include anaerobic, aerobic, facultative lagoons, the activated sludge process and trickling filters [34]. Generally, the mechanisms of biological treatment are a function of bacterial consortium to break down organic waste.

2.1. Anaerobic Treatment Anaerobic treatment technology has proven to be a vital area of research in the management of organic waste. This is because the technology tends to offset the setbacks exhibited by aerobic and physicochemical methods [35]. Considering the portion of the industry’s waste and its by-products that have recovery potential for direct reuse, including nutrients and methane gas, anaerobic systems are a suitable technology for handling high-strength industrial wastewater such as swine and SWW. It is seen in the discharged effluent consistency, material recovery, energy generation, and sludge output, handling, and storage [36]. The biogas composition consists of methane (55–70%) and carbon dioxide (30–45%) under strictly anaerobic conditions. Other contaminants are nitrogen (0–15%), oxygen (0–3%), water (1–5%), hydrocarbons (0–200 mg m−3), ammonia (0–100 ppmv) and siloxane (0–41 mg Si m−3)[37]. Typical anaerobic digestion systems include anaerobic lagoon (AL), anaerobic filter (AF), anaerobic baffled reactor (ABR), and upflow anaerobic sludge blanket reactor (UASB).

2.1.1. Anaerobic Lagoon Anaerobic lagoons (ALs) have been widely applied in the degradation of wastewater, especially in developing countries with hot weather. The method used largely depends on the climate, location, availability of land, and proximity to urban areas [9]. The inﬂuent is SustainabilitySustainability 20212021,, 1313,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1919

oxygenoxygen (0(0––3%),3%), waterwater (1(1––5%),5%), hydrocarbonshydrocarbons (0(0––200200 mgmg mm−−33),), ammoniaammonia (0(0––100100 ppmv)ppmv) andand siloxanesiloxane (0(0––4141 mgmg SiSi mm−−33)) [37].[37]. TypicalTypical anaerobicanaerobic digestiondigestion systemssystems includeinclude anaerobicanaerobic la-la- goongoon (AL),(AL), anaerobicanaerobic filterfilter (AF),(AF), anaerobicanaerobic baffledbaffled reactorreactor (ABR),(ABR), andand upflupflowow anaerobicanaerobic sludgesludge blanketblanket reactorreactor (UASB).(UASB).

2.1.1.2.1.1. AnaerobicAnaerobic LagoonLagoon

Sustainability 2021, 13, 4656 AnaerobicAnaerobic lagoonslagoons (ALs)(ALs) havehave beenbeen widelywidely appliedapplied inin thethe degradationdegradation ofof7 of wastewater,wastewater, 20 especiallyespecially inin developingdeveloping countriescountries withwith hothot weather.weather. TheThe methodmethod usedused largelylargely dependsdepends onon thethe climate,climate, location,location, availabilityavailability ofof land,land, andand proximityproximity toto urbanurban areasareas [9].[9]. TheThe influentinfluent is usually introduced through the bottom of the system and is not mechanical mixed. A usuallyis usually introduced introduced through through the bottom the ofbottom the system of the and system is not mechanicaland is not mixed. mechanical A layer mixed. A layer of scum frequently forms on the surface of the lagoon, ensuring the system is con- oflayer scum of frequently scum frequently forms on forms the surface on the of surface the lagoon, of the ensuring lagoon, the ensuring system is the conﬁned system is con- tofinefine anaerobicdd toto anaerobicanaerobic conditions conditionsconditions with low withwith heat loss.lowlow Figureheatheat loss.loss.5 showed FigureFigure a typical55 showedshowed anaerobic aa typicaltypical lagoon. anaerobicanaerobic la-la- Accordinggoon.goon. AccordingAccording to the literature toto thethe [literatureliterature38,39], the [38,39],[38,39], COD, BOD, thethe COD, andCOD, TSS BOD,BOD, removal andand efﬁciency TSSTSS removalremoval of a typical efficiencyefficiency ofof aa ALtypicaltypical with aALAL depth withwith of aa 3–5 depthdepth m and ofof a33– hydraulic–55 mm andand retentionaa hydraulichydraulic time retentionretention of 5–10 days timetime were ofof 55 found––1010 daysdays as 96%, werewere ffoundound 97%asas 96,96, and 9797 95%, andand respectively. 9595%,%, respectivelyrespectively..

Figure 5. Anaerobic lagoon for wastewater treatment [40]. FigureFigure 5. 5.Anaerobic Anaerobic lagoon lagoon for wastewaterfor wastewater treatment treatment [40]. [40].

However,However,However, this thisthis system’s systemsystem pitfalls’s’s pitfallspitfalls include includeinclude odour odourodour generation generationgeneration and weather andand weatherweather dependency, dependendependency,cy, coupledcoupledcoupled with withwith a longaa longlong HRT HRTHRT and andand requiring requiringrequiring a large aa large arealarge of areaarea land ofof to landland operate. toto operateoperate Thus, to.. Thus,Thus, reduce toto reducereduce odourodourodour and andand smell, smell,smell, the thethe synthetic syntheticsynthetic ﬂoating floatingfloating cover covercover is normally isis normallynormally employed employedemployed to collect toto biogas collectcollect and biogasbiogas andand trap the odour, as shown in Figure6. traptrap thethe odourodour,, asas shownshown inin FigureFigure 6.6.

FigureFigureFigure 6. 6.6.Anaerobic AnaerobicAnaerobic lagoon lagoonlagoon with withwith cover covercover [41]. [41][41]..

Moreover,Moreover,Moreover, the thethe synthetic syntheticsynthetic cover covercover must bemustmust durable bebe durabledurable and able andand to resist ableable change toto resistresist in temperature, changechange inin tempera-tempera- or ice and snow accumulation [9]. ALs are frequently the preferred method of treating SWW ture,ture, oror iceice andand snowsnow accumulationaccumulation [9].[9]. ALsALs areare frequentlyfrequently thethe preferredpreferred methodmethod ofof treat-treat- dueing to SWW their simplicitydue to their as well simplicity as their lowas well operational as their and low maintenance operational costs, and especially maintenance in costs, developinging SWW countriesdue to their [39]. simplicity as well as their low operational and maintenance costs, especiallyespecially inin developingdeveloping countriescountries [39].[39]. 2.1.2. Anaerobic Filters Anaerobic filters are usually run in upflow mode, as the system has a lower risk of washing out the fixed biomass, as shown in Figure7. Sustainability 2021, 13, x FOR PEER REVIEW 8 of 19

2.1.2. Anaerobic Filters Anaerobic filters are usually run in upflow mode, as the system has a lower risk of

Sustainability 2021, 13, 4656 washing out the fixed biomass, as shown in Figure 7. 8 of 20

FigureFigure 7. Anaerobic 7. Anaerobic filter for wastewater filter for treatment wastewater [42]. treatment [42]. In order to ensure an even flow regime, the water level must cover the filter media by at least 0.3In m. order Hydraulic to retentionensure time an (HRT) even is the flow most regime, important design the parameterwater lev to el must cover the filter media influence filter efficiency. For bacteria to grow, the ideal filter should have a large area, with poresby smallat least enough 0.3 to m. avoid Hydraulic clogging. The retention surface area ensures time increased(HRT) contactis the which most important design parameter ultimatelyto influence degrades filter it between efficiency. the organic matterFor bacteria and the attached to grow, biomass. the Ideally, ideal the filter should have a large area, material must occupy a surface area of 90 to 300 m2 per m3 of the volume of the reactor. Thewith typical pores filter content small sizes enough vary from to 12 toavoid 55 mm incloggi diameter.ng. Widely The used surface products area ensures increased contact includewhich dirt, ultimately crushed stones degrades or bricks, cinder, it pumice,between and speciallythe organic shaped plastic matter parts, and the attached biomass. Ide- depending on local availability. The systems are used for the secondary treatment of SWW toally, achieve the high material solids removal must and biogas occupy production. a surface These systems area usually of 90 work to in 300 series m2 per m3 of the volume of the and have a fixed bed biological reactor coupled with a filtration chamber. When the SWW flowsreactor. through T thehe filtration typical chambers, filter large conten and mediumt sizes suspended vary particlesfrom are12 confinedto 55 mm in diameter. Widely used within;products then, the include active biomass dirt, attached crushed to the surface stones of the or filter bricks, degrades cinder, the particulate pumice, and specially shaped plas- organic matter [9]. Gannoun et al. [43] examined the performance of upflow anaerobic filterstic parts, (UAFs) treating depending SWW at mesophilicon local and availability. thermophilic temperatures. The systems The results are used for the secondary treat- showedment that of atSWW an organic to achieve loading rate high (OLR) solids of 9 g/L/d, removal the COD removaland biogas efficiency production. These systems usu- was 90% at mesophilic, and only 72% was achieved at the thermophilic condition. On theally other work hand, thein mesophilicseries and (35◦C) have treatment a fixed of SWW bed at a highbiological organic loading reactor rate of coupled with a filtration cham- OLRber. 10.05 When kg/m3 daythe and SWW HRT of flows 12 h was through evaluated by the Rajakumar filtration et al. [44 chambers,]. The system large and medium suspended recorded a COD removal rate of 79% with a varied methane production between 46 and 56%particles on the average. are Theconfined experiment within; of Stets et then, al. [45] evaluated the active the influence biomass of substrate attached to the surface of the filter characteristics,degrades microorganismsthe particulate present organic in the sludge, matter and the[9]. supporting Gannoun media et in al. AF. [43] examined the performance The results showed a maximum COD and TN removal efficiency of 80 and 90% at an HRT ofof 1 day. upflow The major anaerobic drawback of filters anaerobic (UAFs) baffle reactor treating is the need SWW for relatively at mesop higher hilic and thermophilic temper- temperaturesatures. The for optimum results service, showed but this that is not anat obstacle an organic in tropical loading countries. rate (OLR) of 9 g/L/d, the COD re- 2.1.3.moval Anaerobic efficiency Baffled Reactor was 90% at mesophilic, and only 72% was achieved at the thermophilic condition.Anaerobic baffled On reactorsthe other (ABRs) hand, consist ofthe a series mesophilic of compartments (35°C) with inlettreatment and of SWW at a high organic outlet, in which SWW flows in from beneath and above. The reactor is commonly referred 3 toloading as an optimized rate version of OLR of a septic10.05 tank, kg/m and theday diagrammatic and HRT representation of 12 h ofwas the evaluated by Rajakumar et al. reactor[44]. design The andsystem its characteristic record dimensionsed a COD is shown removal Figure8. rate of 79% with a varied methane production between 46 and 56% on the average. The experiment of Stets et al. [45] evaluated the influence of substrate characteristics, microorganisms present in the sludge, and the supporting media in AF. The results showed a maximum COD and TN removal efficiency of 80 and 90% at an HRT of 1 day. The major drawback of anaerobic baffle reactor is the need for relatively higher temperatures for optimum service, but this is not an obstacle in tropical countries.

2.1.3. Anaerobic Baffled Reactor Anaerobic baffled reactors (ABRs) consist of a series of compartments with inlet and outlet, in which SWW flows in from beneath and above. The reactor is commonly referred to as an optimized version of a septic tank, and the diagrammatic representation of the reactor design and its characteristic dimensions is shown Figure 8. SustainabilitySustainability 20212021,, 1313,, x 4656 FOR PEER REVIEW 9 9of of 19 20

FiFiguregure 8. 8. AnaerobicAnaerobic baffled bafﬂed reactor reactor [46] [46]..

TheThe purpose purpose of usingusing thethe anaerobic anaerobic baffled baffled reactor reactor is tois provideto provide the enhancedthe enhanced removal removaland digestion and digestion of organic of organic matter matter as well as as well of microorganisms as of microorganisms present present in the influent.in the influ- The ent.design The objectivedesign objective was to increasewas to increase the contact the con timetact between time between the suspended the suspended or dissolved or dis- solvedcontaminants contaminants and biomass and biomass and to and minimize to minimize the amount the amount of sludge of sludge washout washout in the in ABR the ABReffluent. effluent. This This can becan achieved be achieved by maximizing by maximizing the hydraulic the hydraulic retention retention time, time, the number the num- of berpasses of passes through through the sludge the sludge bed (i.e., bed the (i.e., number the number of compartments), of compartments), and the and upflow the upflow rate to ratereduce to reduce the transport the transport of solids of withinsolids within processing processing and capital and costcapital constraints cost constraints as determined as de- terminedby solid retention. by solid retention. Two six-compartment Two six-compartment anaerobic anaerobic baffled reactors baffled reactors to be installed to be inin stalledseries are in series usually are designed usually todesigned achieve to maximum achieve maximum treatment treatmentrates. This rates. engineered This engi- two neeredsix-compartment two six-compartment ABR offers 96ABR h (48 offers h for 96 each h (48 ABR) h for hydraulic each ABR) retention hydraulic period retention which pe- by riodfar waswhich higher by far than was the higher 48–92 than h ranges the 48– for92 highh ranges peak-flow for high output peak-flow levels output and the levels 20–60 and h, thewhich 20–60 allowed h, which high-performance allowed high-performance treatment for treatment domestic for wastewater. domestic Thewastewater. peak up-flow The peakrate ofup 0.54-flow m/h rate wasof 0.54 proposed m/h was by proposed Foxon and by Buckley Foxon and [47], Buckley and peak [47], flow and factor peak offlow 1.8 factorresulted of 1.8 in anresulted upflow in rate an ofupflow 0.30 m/h. rate of This 0.30 value m/h. corresponds This value tocorresponds the one suggested to the one by suggestedTilley et al. by [42 Tilley], which et al. is <0.6[42], m/h.which The is < study0.6 m/ ofh. Ku¸sçuandThe study Sponzaof Kuşçu [48 and] revealed Sponza that [48] a revealedsignificant that improvement a significant in COD improvement and BOD inremovals COD and up to BOD 90% wasremovals achieved up in to the 90% upflow was achievedcompartment. in the Aupflow laboratory-scale compartment. study A oflaboratory the performance-scale study of combine of the performance ABR and UV/H of com-2O2 treating SWW with a total organic carbon (TOC) concentration of 973 mg/L exhibited high bine ABR and UV/H2O2 treating SWW with a total organic carbon (TOC) concentration of 973organic mg/L carbon exhibited removal high efficiencyorganic carbon up to removal 95% [49]. efficiency One major up drawbackto 95% [49]. of thisOne typemajor of reactor is that the system does not have auxiliary mechanisms for the retention of biomass, drawback of this type of reactor is that the system does not have auxiliary mechanisms in the case of large variations and extreme peaks of the influential flow. for the retention of biomass, in the case of large variations and extreme peaks of the influ- ential2.1.4. flow. Upflow Anaerobic Sludge Blanket Reactor

2.1.4. TheUpflow development Anaerobic ofSludge the UASB Blanket technology Reactor dated back to the late 1970s, and was initially developed for the anaerobic treatment of liquid waste streams with a high con- centrationThe development of COD (1.0 of tothe 200 UASB g COD technology L−1) and dated low back solid to content the late [ 501970s,,51]. and The was upflow ini- tiallyanaerobic developed sludge for blanket the anaerobic (UASB) treatm reactorent is of a tankliquid with waste a sludge streams bed with occupying a high concen- half or −1 trationless the of volume COD (1.0 of to the 200 total g COD tank L from) and the low bottom solid content of the tank. [50,51]. UASB The reactorupflow consistsanaerobic of sludgethree zones: blanket the (UASB) sludge reactor zone containing is a tank thewith biomass, a sludge substrate-like bed occupying SWW, half and or less the gasthe zonevol- umeabove of thethe substrate total tank [52 from,53]. the As thebottom name of implies, the tank. upflow, UASB the reactor SWW consists enters fromof three the bottomzones: theand sludge flows zone upward containing with a highthe biomass, or low velocitysubstrate through-like SWW, the sludgeand the blanket, gas zone which above then the substrateexits from [52,53]. the top As as the an name effluent implies as illustrated, upflow, the in SWW Figure enters9. Depending from the onbottom the prevailing and flows upwardparameters, with literaturea high or low have velocity reported through a satisfactory the sludge performance blanket, which of the then UASB exits reactor from the in topdegrading as an effluent SWW. as illustrated in Figure 9. Depending on the prevailing parameters, literature have reported a satisfactory performance of the UASB reactor in degrading SWW. Sustainability 2021, 13, 4656 10 of 20 Sustainability 2021, 13, x FOR PEER REVIEW 10 of 19

FigureFigure 9.9. ConceptualConceptual diagramdiagram ofof upﬂowupflow anaerobicanaerobic sludgesludge blanketblanket (UASB)(UASB)[ [54].54].

TheThe manymany advantagesadvantages of of UASB UASB reactors reactors include include less less sludge sludge production, production, energy energy recov- re- ery,covery, and theand overall the overall lowcost low of cost application of application [55]. Moreover, [55]. Moreover, the bacteria the bacteria can withstand can withstand a long perioda long ofperi starvationod of starvation without without dying, and dying only, and one only discharge one discharge of sludge of is sludge required is perrequired year forper a year UASB for reactor a UASB with reactor around with 4 m around high. Tropical4 m high. countries Tropical stand countries to benefit stand more to benefit in the usemore of in the the UASB use reactorof the UASB because reactor they work because better they at mesophilicwork better conditions. at mesophilic The researchconditions. of CalderaThe research et al. of [56 Caldera] demonstrated et al. [56] that demonstrated a high COD that removal a high efficiency COD removal of up toefficiency 94.31% fromof up ato UASB 94.31% reactor from a treating UASB reactor SWW under treating mesophilic SWW under condition. mesophilic The substratecondition. concentration The substrate variedconcentration from 1820 varie tod 12,790 from 1820 mg/L, to and12,790 the mg/L, experiment and the lasted experiment for 90 dayslasted at for HRT 90 ofdays 24 h.at InHRT another of 24 development,h. In another development, Chávez et al. Chávez [57] reported et al. [57] the 95%reported BOD the removal 95% BOD efficiency removal of UASBefficiency treating of UASB slaughterhouse treating slaughterhouse waste at an optimum waste at OLRan optimum 31,000 mg/L OLR 31,000 under mg/L mesophilic under 3 conditionsmesophilic at conditions HRT 3.5 and at HRT 4.5 h. 3.5 The and work 4.5 ofh. Miranda The work on of the Miranda 800 m full-scaleon the 800 UASB m3 full reactor-scale treatingUASB reactor SWW withtreating an influentSWW with of COD an influent concentration of COD in con thecentration range of 1400–3600in the range mg/L of 1400 and– oil3600 and mg/L grease and content oil and between grease content 413 and bet 645ween mg/L, 413 respectively.and 645 mg/L, The respectively. results of their The experi-results mentof their revealed experiment that aroundrevealed 70–92% that around COD 70 and–92 27–58%% COD oiland and 27– grease58% oil removal and grease efficiencies. removal Moreover,efficiencies. an Moreover, optimum an COD optimum removal COD efficiency removal of 90%efficiency was also of 90% revealed was also in the revealed study ofin Rajakumarthe study of and Rajakumar Meenambal, and Meenambal, [58] at an HRT [58] of at 10 an h, HRT varying of 10 the h, CODvarying concentration the COD concen- from 3000tration to 4800from mg/L3000 to in 4800 the UASB mg/L reactor.in the UASB Mijalova reactor. et al. Mijalova [59] analysed et al. the[59] outputanalysed of athe UASB out- reactorput of a treating UASB reactor SWW after treating solid SWW separation after solid under separation the ambient under condition. the ambient It was condition. reported thatIt was the reported efficiencies that of the COD efficiencies removal of improved COD removal in relation improved to OLRs. in relation With an to influent OLRs. CODWith concentrationan influent COD of 3437 concentration mg/L, the of system 3437 mg/L, recorded the asystem high COD recorded removal a hi efficiencygh COD removal of 90%. Whileefficiency UASB of reactors90%. While are foundUASB toreactors be effective are found for SWW to be treatment,effective for compliance SWW treatment, with current com- waterpliance quality with standardscurrent water for water quality body standards discharge for requires water abody post-treatment discharge process.requires Tablea post2 - shows the review of the performance of previous works on UASB reactors treating SWW treatment process. Table 2 shows the review of the performance of previous works on and other wastewater. However, the system shortfall of sludge washout at elevated upflow UASB reactors treating SWW and other wastewater. However, the system shortfall of velocity and the slow-growing methanogenic bacteria. The performance of various UASB sludge washout at elevated upflow velocity and the slow-growing methanogenic bacteria. reactors treating slaughterhouse and other wastewaters are shown in Tables2 and3. The performance of various UASB reactors treating slaughterhouse and other wastewaters are shown in Tables 2 and 3. Sustainability 2021, 13, 4656 11 of 20

Table 2. The performance comparison of different UASB reactor treating slaughterhouse wastewater.

Temperature SMP Type of Substrate OLR HRT (h) ◦ %COD Removal Biogas Production −1 Scale Reference ( C) (L g CODadded) Slaughterhouse 0.2–1.4 12 24–35 ◦C 30–62% 3.45 L/d NR Lab [60] wastewater kg COD/m3d−1 Slaughterhouse 1.46 to 2.43 kg 18–27 25 ◦C 70–92% NR NR Full [61] wastewater COD/m3d−1 Slaughterhouse 0.64 - 2.95 kg NR 35 ◦C 58.4% 270 mL/d NR Lab [62] wastewater COD/m3d−1

Slaughterhouse 13–39 kg ◦ 200–280 LCH4/kg 3 −1 2–7 33 C 75–90% NR Pilot [63] wastewater SCOD/m d SCOD removed 0.020 ± 70.013 0.239 ± 70.095 Slaughterhouse 4–15 kg COD/m3d−1 0.88, 0.71 0.44, 0.30 20.9–25 ◦C 90% 0.039 ± 70.010 0.266 ± 70.005 m3/kg Lab [59] wastewater 3 0.095 ± 70.008 m /d COD removed Poultry slaughterhouse 2.1 kg COD/m3d−1 1–5 NR >80% NR NR Lab [64] wastewater slaughterhouse 1.27–17 kg COD/ 4–0.3 35 ◦C NR 0.680–3.790 L.L−1. day−1 NR Lab [65] wastewater m3d−1 Poultry 0.24 m3CH / slaughterhouse 15 kg COD/m3d−1 24, 16, 12, 10 and 8 29–35 ◦C 78% 20.3 L/d 4 Lab [44] kg COD wastewater removed 0.09 ± 0.03 to Slaughterhouse 0.32, 0.51, 1.16 and 22, 14, 6 and 3 29.6 ± 1.40 ◦C 43.39–84.54% 143.9 m3 0.22 ± 0.02 m3/ Pilot [66] wastewater 2.31 kg COD/m3d−1 kgCOD removed Slaughterhouse 1.2 kg COD m−3 d−1 24 30 ± 1 ◦C 70% NR NR Lab [67] wastewater Slaughterhouse 100 mL 0.54 15.6 35 ± 1 ◦C 50.9 NR Lab [68] wastewater CH4/gCODadded Sustainability 2021, 13, 4656 12 of 20

Table 3. UASB performances on various types of wastewater.

HRT OLR Type of Substrate Temperature Inﬂuent COD Biogas Produced COD Removal (%) References (h) (d) Ambient temperature Low strength wastewater 500 mg/L 3 4 kg COD/m3/d 141 L/kg COD 90–92% [69] (20–35 ◦C) removed 0.34 m3CH / Domestic wastewater Ambient temperature - 7.6 1.21 kg COD/m3/d 4 85% [70] g COD removed ◦ Wheat straw stillage 55 C 70 g/L 48 17.1 g COD/L/d 154.8 mL CH4/g COD 76% [71] Composite chemical 0.3 m3CH /kg 29 ± 2 ◦C 6600 mg/L 37 4.25 kg COD/m3/d 4 62% [72] wastewater COD removed 0.23 L CH / Potato leachate wastewater 37 ◦C 20 g/L 6.1 g COD/L/d 4 93 ± 5.3% [73] g COD degraded 0.23 ± 0.03 NL CH /g Seaweed leachate 37 ± 1 ◦C 7.3 ± 1.1 g/L 88.8 2.9 g COD/L/d 4 -[74] CODadded High-strength municipal 30 ◦C 1200 mg/L 4 7.2 kg COD/m3/day 306.6 mL CH /g COD 85% [75] wastewater 4 removed 250 ± 6 mL CH / Potato juice 37 ◦C 25.2 g/L 240 2.5 g COD/L/d 4 -[76] gVS added High salinity wastewater from 30 ± 2 ◦C 350–640 mg/L 48 0.23 kg COD/m3/d - 65.08% [77] heavy oil production Ambient temperature Low strength wastewater 700–1000 mg/L - 1.293 kg COD/m3/d 457 L/kg COD 90.8% [78] (24–35 ◦C) removal Sustainability 2021, 13, 4656 13 of 20

A literature review on the anaerobic digestion process was carried out to identify the main concepts and operating parameters associated with the upflow anaerobic sludge blanket reactors, as the selected technology and the issues relevant to the investigations (Tables2 and3 ). The review summarizes the main findings related to COD removal efficiencies and the biogas production of the UASB reactors treating slaughterhouse and slaughterhouse-related wastewater. A considerable amount of attention was given to the effects of OLR and HRT on the efficiency of the systems which are potentially the key parameters for the digestion of slaughterhouse and other organic wastewater. A laboratory scale study of the anaerobic digestion of slaughterhouse wastewater by Chollom et al. [60] showed 30–62% COD removal efficiency. It can be seen that, the COD removal efficiency and the biogas production were low. These could be due the low temperature and the HRT. Similarly, Batubara et al. [62], Nacheva et al. [59] and Del Nery et al. [64] studied the anaerobic digestion of slaughterhouse wastewater at the laboratory scale. How- ever, it was observed that the systems were highly characterized by long HRT and low biogas production. On the other hand, a pilot scale investigation showed a high COD removal efficiency [63,66]. Conversely, the work of Worku [62] revealed a very low specific methane production (SMP), and this could be attributed to the low COD of the effluent (COD = 7049.07 ± 306.42 mg/L). The application of the UASB reactor achieved considerable success in the treatment of a wide range of other organic industrial effluents including low- and high-strength domestic wastewater as depicted in Table3. However, the systems were characterized by long HRT, although the study of Singh et al. [69] and Hazrati and Shayegan [75], showed a lower HRT and high COD removal efficiency, but the influent COD concentration was very low as compared to other wastewater presented in Table3. Therefore, there is the need to modify the UASB reactor to treat high strength wastewater at a higher OLR and short HRT and to also comply with the stringent environment regulations.

2.1.5. Suspended and Attached Growth Process The waste flows through and around free-floating microorganisms in a suspended- growth system, such as activated sludge processes (Figure 10A,B) (also aerated lagoons and aerobic digestion), accumulating into biological flocks which then settle out of the wastewater. the influent wastewater characteristics such as (COD), total N, and total P and operating parameters like HRT, SRT, dissolved oxygen (DO), return activated sludge (RAS), and mixed liquor recirculation (MLR) flow rate have significant impacts on the performance of anaerobic, anoxic and aerobic system (Figure 10A). However, the compe- tition for organic substrates among the bacterial population, there is a concern about the adverse effects of the returning sludge on the growth of the bacteria, which prefer to grow under alternating anaerobic and aerobic conditions. Therefore, to overcome the inherent drawbacks of the anaerobic, anoxic and aerobic processes, a reverse anoxic, anaerobic and aerobic process (Figure 10B) is believe to improve the performance of the bacteria through aerobic conditions. The microbial mass is retained as flocs in suspended growth systems in the mixed liquor of the reactor. Mechanical mixers or gas injection hold these flocs in suspension with agitation. The air in aerobic processes and biogas in anaerobic systems will normally be the latter. Agitation facilitates intimate interaction between the substrate and the biomass. Microbes are attached to the support medium in a thin layer in biofilm systems. The latter can be a static bed or a moving bed. Fixed or stationary beds are usually moulded plastic or gravel shapes, while moving beds may include granular activated carbon or sand grains. These beds of support medium may be submerged in a mixed liqueur during the operation of the reactor or otherwise exposed to the air and wastewater. The bulk of the aerobic systems used in wastewater treatment plants are suspended growth systems. Other examples of suspended growth systems include the aerated lagoon, oxidation ditch, and batch sequencing reactor. However, biofilms with suspended growth can sometimes be integrated into such aerobic systems. Attached growth systems (also known as fixed- Sustainability 2021, 13, x FOR PEER REVIEW 13 of 19

A literature review on the anaerobic digestion process was carried out to identify the main concepts and operating parameters associated with the upflow anaerobic sludge blanket reactors, as the selected technology and the issues relevant to the investigations (Tables 2 and 3). The review summarizes the main findings related to COD removal efficiencies and the biogas production of the UASB reactors treating slaughterhouse and slaughterhouse-related wastewater. A considerable amount of attention was given to the effects of OLR and HRT on the efficiency of the systems which are potentially the key parameters for the digestion of slaughterhouse and other organic wastewater. A laboratory scale study of the anaerobic digestion of slaughterhouse wastewater by Chollom et al. [60] showed 30–62% COD removal efficiency. It can be seen that, the COD removal efficiency and the biogas production were low. These could be due the low temperature and the HRT. Similarly, Batubara et al. [62], Nacheva et al. [59] and Del Nery et al. [64] studied the anaerobic digestion of slaughterhouse wastewater at the laboratory scale. However, it was observed that the systems were highly characterized by long HRT and low biogas production. On the other hand, a pilot scale investigation showed a high COD removal efficiency [63,66]. Conversely, the work of Worku [62] revealed a very low specific methane production (SMP), and this could be attributed to the low COD of the effluent (COD = 7049.07 ± 306.42 mg/L). The application of the UASB reactor achieved considerable success in the treatment of a wide range of other organic industrial effluents including low- and high-strength domestic wastewater as depicted in Table 3. However, the systems were characterized by long HRT, although the study of Singh et al. [69] and Hazrati and Shayegan, [75], showed a lower HRT and high COD removal efficiency, but the influent COD concentration was very low as compared to other wastewater presented in Table 3. Therefore, there is the need to modify the UASB reactor to treat high strength wastewater at a higher OLR and short HRT and to also comply with the stringent environment regulations.

Sustainability 2021, 13, 4656 14 of 20 2.1.5. Suspended and Attached Growth Process The waste flows through and around free-floating microorganisms in a suspended- ﬁlmgrowth processes) system, are such processes as activated for the treatment sludge processes of biological (Figure wastewater 10) (also with aerated the biomass lagoons and attachedaerobic to digestion), certain types accumulating of media. Figure into 11 a biological shows the laboratory flocks which scale attachedthen settle growth out of the system,wastewater. while Figure 11b represents a pilot-scale attached growth system, likewise.

Sustainability 2021, 13, x FOR PEER REVIEW 14 of 19

latter can be a static bed or a moving bed. Fixed or stationary beds are usually moulded plasticlatter canor gravelbe a static shapes, bed whileor a moving moving bed. beds Fixed may or include stationary granular beds areactivated usually carbon moulded or sandplastic grains. or gravel These shapes, beds of whilesupport moving medium beds may may be submergedinclude granular in a mixed activated liqueur carbon during or thesand operation grains. These of the beds reactor of support or otherwise medium exposed may be to submergedthe air and inwastewater. a mixed liqueur The bulk during of thethe aerobic operation systems of the used reactor in wastewater or otherwise treatment exposed plantsto the areair andsuspended wastewater. growth The systems. bulk of Otherthe aerobic examples systems of suspended used in wastewater growth systems treatment include plants the aeratedare suspended lagoon, growthoxidation systems. ditch, andOther batch examples sequencing of suspended reactor. However,growth systems biofilms include with thesuspended aerated lagoon,growth oxidationcan sometimes ditch,

beand integrated batch sequencing into such reactor. aerobic However,systems. Attached biofilms wigrowthth suspended systems growth(also known can sometimes as fixed- FigureFigurefilmbe integrated processes) 10. 10.Schematic Conventional into are processessuch of two activatedaerobic conventional for systems.the sludge treatment activated processAttached of sludge [79]biological growth. processes systemswastewater [79]. (also It’s thewith known same the diagram:biomassas fixed - (attachedAfilm) and processes) (B ).to The certain only are types differenceprocesses of media. is for that the theFigure treatment mixed 11 liquor represents of biological recirculation the schematicwastewater (MLR) is diagram returned with the toof biomass aerobicthe at- conditiontachedattachedThe growth ( Atomicrobial) andcertain system, sometimes types mass while of is is tomedia.: retained return Figure anoxic as (11floB). representscs in suspended the schematic growth diagram systems of thein theat- mixed liquortached ofgrowth the reactor. system, Mechanicalwhile: mixers or gas injection hold these flocs in suspension with agitation. The air in aerobic processes and biogas in anaerobic systems will normally be the latter. Agitation facilitates intimate interaction between the substrate and the biomass. Microbes are attached to the support medium in a thin layer in biofilm systems. The

Sustainability 2021, 13, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/sustainability

FigureFigure 11.11. AAttached schematic growth diagram system of attached [80]. growth system [80]. Figure represents attached growth systemFigure in 11. (a Attached) lab-scale growth and (b) system pilot-scale. [80]. Figure 12 shows the difference between the suspended and the attached growths. The growthFigureFigure 12 formed12 shows shows in the thethe difference mediadifference is betweena mixture between the of the suspended mainly suspended aerobic and theandmicroorganisms. attached the attached growths. growths. The growthThe growth formed formed in the in media the media is a mixture is a mixture of mainly of mainly aerobic aerobic microorganisms. microorganisms.

Figure 12. Typical examples of suspended and attached growth system [81] . FigureFigure 12. 12.Typical Typical examples examples of of suspended suspended and and attached attached growth growth system system [81 [81]]. . These are similar to those found in other secondary biological treatment processes. The microorganismsThese are similar include to those ciliates, found rotifers, in other nematodes, secondary andbiological many otherstreatment that processes. are free- swimmingThe microorganisms and stalking. include Attached ciliates, growth rotifers, processes nematodes, are easy and to many operate others and that resistant are free to - shockswimming loads andbut arestalking. less versatile Attached than growth activated processes sludge are processes easy to foroperate process and control. resistant to shock loads but are less versatile than activated sludge processes for process control. 2.1.6. Summary of Biological Treatment Methods 2.1.6.Table Summary 4 summarizes of Biological the advan Treatmenttages andMethods disadvantages of the different physicochemical treatmentTable 4 summarizes methods of theslaughterhouse advantages and wastewater. disadvantages of the different physicochemical treatment methods of slaughterhouse wastewater.

Sustainability 2021, 13, 4656 15 of 20

These are similar to those found in other secondary biological treatment processes. The microorganisms include ciliates, rotifers, nematodes, and many others that are free- swimming and stalking. Attached growth processes are easy to operate and resistant to shock loads but are less versatile than activated sludge processes for process control.

2.1.6. Summary of Biological Treatment Methods Table4 summarizes the advantages and disadvantages of the different physicochemical treatment methods of slaughterhouse wastewater.

Table 4. Advantages and disadvantages of biological treatment methods.

Methods Advantage Disadvantage

It is the most preferred method of treating slaughterhouse wastewater (SWW) due to its simplicity, low cost of operation and Depends largely on the climate, location, maintenance. availability of land, and proximity to urban areas. No mechanical mixing is required. A layer of scum frequently forms on the Anaerobic lagoon A typical AL with a depth of 3–5 m and a (AL) hydraulic retention time (HRT) of 5–10 days surface. usually achieve COD, BOD, and TSS removal Long HRT. efficiency of up to 96%, 97% and 95%. The synthetic cover may experience leakages with time due to fluctuation in temperature. Odour and smell reduction is achieved by employing synthetic floating cover to collect biogas and trap the odour.

Have 3 chambers with filters and the active Relatively high temperature is required. biomass attached to the surface of the filter Sludge sedimentation. Anaerobic filters degrades the particulate organic matter. The systems are used for secondary treatment

(AF) Runs in upﬂow mode, hence mechanical of SWW to achieve high solids removal and mixing is not required. biogas production. The ﬁlters can easily clog.

Increases the contact time between suspended Lacks biomass retention mechanism. Anaerobic bafﬂed or dissolved contaminants and biomass and Long HRT. reactor (ABF) minimizes the amount of sludge washout. Biomass could easily washout at peak ﬂow and short HRT.

Less sludge production. Long start-up period due to slow growing Energy and materials recovery. methanogenic bacteria. Upﬂow anaerobic Can operate at higher organic loading rate sludge blanket (OLR). Sludge washout at low HRT. Scum formation on the substrate surface. reactor Required small reactor volume and space for installation. Efﬂuent post treatment.

Low operation and maintenance cost.

Mainly aerobic bacteria, hence no energy is Commonly applied method for a large volume produced. of wastewater. Suspended growth Require large space. Biomass recirculation simplified the (activated sludge High cost of operation and maintenance. continuous operation. process) Not flexible (in case of change in waste Good effluent quality. concentration).

Sludge disposal is required on large scale.

The attached growth processes are low maintenance, low energy requirements, and, Mostly suitable for the treatment of Attached growth overall, less technology involved. wastewater for small communities. Larger area requirement, ineffective in cold process Very effective for biochemical oxygen demand (BOD) removal, nitriﬁcation, and weather, and creates odour problems. denitriﬁcation Sustainability 2021, 13, 4656 16 of 20

2.2. Concluding Remarks The literature review revealed that anaerobic digestion appeared as a promising technology for the treatment of low- and high-strength slaughterhouse wastewater, although it is a complex and sensitive process. The operation of the anaerobic reactor is highly dependent on the temperature, pH, hydraulic retention time, and loading rates as well as wastewater and biomass characteristics. It was also found that, for good substrate degradation, anaerobic reactors’ optimal temperature conditions include psychrophilic (<25 ◦C), mesophilic (25–40 ◦C) and thermophilic (>45 ◦C). However, most of the studies showed that the reactor performance was more stable at mesophilic condition (25–40 ◦C). Likewise, stable pH condition usually exists between 6.5 and 7.5. Among the various anaerobic reactors, UASB reactor showed high biogas production and COD removal efficiency at high OLR. The system is also characterized by low sludge production compared to other physicochemical treatment methods, where less energy is applied and high energy is generated and the overall cost of operation and maintenance is lower. While conventional UASB reactors appear to be a promising choice for the recovery of energy from organic wastewater, including slaughterhouse wastewater (SWW), there are potential problems associated with the reactor. These include the long start-up period due to the slowly growing microorganism, sludge washout at low HRT, scum formation on the substrate surface—especially in in the treatment of SWW—and the suspended solid accumulation at high inflow velocity with low HRT. Several researchers have studied the role of attached growth media in increasing the concentration of the microbial population. However, most of the systems were highly characterized by long HRT and low OLR especially in conventional UASB reactors (Table2 ). Furthermore, most of the media used for microbial attachment in anaerobic reactors are usually made of plastic materials with a smooth surface and normally configured in suspended moving media, which leads to the overflow of the media on the substrate surface during influent pumping at higher velocity and these consequently result in insufficient contact between the microbes and the media. Additionally, as a result of the separation between microbes and the media, the microbes could easily washout during effluent discharge. Despite the numerous studies on this subject, none of the previous research has focused on the comparison between the performance of the conventional UASB reactor and UASB reactor with fixed attached growth media with a rough and large surface area that confines the whole sludge zone in a UASB reactor treating high strength cattle slaughterhouse wastewater (CSWW). The literature review has thus identified some key gaps in the knowledge, especially in aerobic and anaerobic treatment processes, and indicated a number of concepts and tools that may be useful in future research. For instance, aerobic processes are highly characterized by high energy demand, the large area of land for installation, huge quantity of sludge production and inefficient small and medium scale industries. Similarly, most anaerobic systems required a long HRT for bacterial growth, and sludge easily washes out along with the large microbial population during effluent discharge and is highly temperature dependent. Therefore, further research on the use of organic or inorganic waste materials or cellulose materials should be conducted to further harness the most cost-effective methods of slaughterhouse wastewater treatment.

Author Contributions: Conceptualization, M.A.M. and S.I.; methodology, M.A.M.; validation, S.I.; formal analysis, M.A.M.; investigation, M.A.M.; resources, S.I.; data curation, S.I.; writing—original draft preparation, M.A.M.; writing—review and editing, S.I.; visualization, S.I.; supervision, S.I. All authors have read and agreed to the published version of the manuscript. Funding: This research was ﬁnancially supported by the Ministry of Higher Education Malaysia through Fundamental Research Grant Scheme (FRGS/2/2014/TK02/UPM/02/6) and Tenaga Na- sional Berhad Research Sdn. Bhd. through Industrial grant (TNBR/Biogas/2019/UPM/6380035). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Sustainability 2021, 13, 4656 17 of 20

Data Availability Statement: Not applicable. Acknowledgments: The authors acknowledge the support received from Ministry of Higher Educa- tion Malaysia. Also, Tenaga Nasional Berhad Research Sdn. Bhd and the Universiti Putra Malaysia, for the preparation, execution, and writing of this article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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