applied sciences

Article Management of Waste Streams from Dairy Manufacturing Operations Using Membrane and Dissolved Air Flotation

Subbiah Nagappan 1, David M. Phinney 2 and Dennis R. Heldman 1,2,*

1 Department of Food, Agriculture and Biological Engineering, 590 Woody Hayes Drive, Columbus, OH 43210, USA; [email protected] 2 Department of Food Science and Technology, 2015 Fyffe road, Columbus, OH 43210, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-614-292-5899

 Received: 27 November 2018; Accepted: 14 December 2018; Published: 19 December 2018 

Featured Application: This article deals with two different methods of in the dairy industry. This article will help industrialists in the dairy world who have problems with their wastewater treatment.

Abstract: Membrane filtration can provide a significant role in the management of waste streams from food manufacturing operations. The objective of this research was to evaluate the reductions in the organic content of waste streams accomplished when using membrane filtration. Reductions in Chemical Oxygen Demand (COD) by membrane filtration were compared to a Dissolved Air Floatation (DAF) system. Membranes with six different pore sizes (200, 20, 8, 4, 0.083, and 0.058 kDa) were evaluated. In addition, the various membrane treatments were applied after the DAF as an additional level of comparison. The DAF treatment provided 75.15 ± 3.95% reduction in COD, and the reduction in COD improved from 85% to 99% as the membrane pore size decreased. When all membranes were used after a DAF pre-treatment, a reduction in COD to less than 1200 ppm in the permeate stream was achieved. These reductions were independent of the COD in the feed stream. The membrane fouling rates were evaluated for the membranes with the four largest pore-sizes membranes. The membranes with 20 kDa pore-size had the lowest fouling rates during extended fouling-rate studies.

Keywords: membrane filtration; fouling; dairy wastewater

1. Introduction Dairy industries consume huge amounts of water, accounting for 33.96% of water consumption in all food industries [1]. Water is consumed in different ways in dairy industries. Examples include; as an ingredient, in clean in place (CIP), as boiler feed, in cooling tower operation, etc. Among these operations, CIP accounts for 38% of the total water consumption in the dairy industry [2]. Due to the large aforementioned point uses of water, the dairy industry generates 0.2 L to 10 L of effluent per L of processed milk [3]. Dairy wastewaters are characterized by high chemical oxygen demand (COD) due to high organic content caused by the presence of fats, proteins and carbohydrates [4]. This high nutrient content in the dairy wastewater is due to dumping dairy products down the drain and cleaning processed equipment and pipes [5,6]. When the highly nutritive effluent from the dairy industry is not treated and dumped into rivers, it causes eutrophication by organic, nitrogen and phosphorous compounds [7,8].

Appl. Sci. 2018, 8, 2694; doi:10.3390/app8122694 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2694 2 of 15

In most municipalities where there is no river close by, the dairy wastewater is sent to a municipal wastewater treatment (MWWT) plant for further treatment, before being discharged into other bodies of water. This gives a final safe layer of treatment. Depending upon the type and volume of products manufactured, municipal wastewater treatment plants set limits on certain water quality parameters (COD, BOD, fats, oils, total solids, etc.). If industries discharge water to the MWWT plant that is over the limit, they receive penalties in terms of surcharges [9]. Hence, treating the wastewater before discharging either into a river or to the municipal treatment plant is necessary to avoid serious environmental and financial impacts. The type and volume of dairy products manufactured varies from one industry to another, causing high variability in the nature of dairy wastewater, making it difficult to choose a particular wastewater treatment method. Current treatment methods for dairy wastewater are aerobic treatment, anaerobic treatment, dissolved air floatation (DAF), activated sludge, clarification, sand bio filters, membrane filtration, advanced oxidation processes, coagulation/electrocoagulation, moving bed biofilm reactors, membrane bioreactors and other treatments. Combined treatments are most effective to tackle the heterogeneity of the wastewater [10,11]. DAF is the most commonly used treatment method for dairy wastewater [12]. CIP consumes the most water in the dairy industry, and thus, also generates the highest volume of effluent among all dairy operations. High concentrations of dissolved salts and high pH in the dairy effluent is due to CIP operation [11]. Therefore, depending upon the operation going on in the dairy plant, the influent to the wastewater treatment plant varies. To tackle this high heterogeneity in the feed, big equilibrium tanks (EQ) are used. These huge tanks collect all the wastewater from the dairy plant over a long period and mix them using pumps to produce a certain degree of homogeneity in the feed to the wastewater treatment plants. A dairy industry in Ohio utilizes dissolved air floatation (DAF) as its wastewater treatment method. This method uses coagulants, flocculants and air bubbles to remove suspended particles from the water. Coagulants like polyaluminum chloride are used todestabilize the suspended particles, and flocculants—like acrylamides— aggregate these destabilized particles into big clusters. These clusters adhere to the surface of the air bubbles and rise to the top of the DAF where it is skimmed off. This type of treatment method is effective against treating wastewaters containing high fat, oil and greases and suspended solids. The disadvantage of this treatment is that it is a very chemical-intensive process. This dairy industry had a COD discharge limit of 1200 ppm to the municipality and it received surcharges, since its wastewater contained significant dissolved solids and the DAF was not effective against treating them. Thus, there was a need for another wastewater treatment. Membrane filtration is not only used as a wastewater treatment method in dairy industries [13], but also for the reclamation and reuse of water [14,15], whey fractionation [16], recovery of cleaning solutions [17] and other purposes [18]. In cross flow membrane filtration, the feed is more highly pressurized than the osmotic pressure across a semipermeable membrane, and the particles that can pass through the membrane pores come out through the circumference as permeates, while the other particles are retained and come out the other end as retentate. Membranes will foul with time due to the accumulation of particles on the pores, and thus, the permeate flux reduces. To recover lost flux, a membrane has to be cleaned from time to time, and, after certain a number if cleanings, replaced [19]. With technology, membranes are manufactured to reduce fouling effects and withstand a wide range of pH and temperatures of the feed. The use of membranes for wastewater treatment is increasing due to the availability of low cost and more versatile pore sized membranes. Due to the various advantages of this treatment method, the influence of membrane filtration on dairy waste streams was studied. The overall objective for this investigation was to evaluate the use of membrane filtration for the management of the wastewater stream from a dairy manufacturing operation. The specific objectives were; (1) to compare the effectiveness of dissolved air floatation (DAF), membrane filtration and combined DAF and membrane filtration for reducing the COD of the waste water stream, (2) to select the appropriate process design for COD reduction of the dairy manufacturing waste stream, based on Appl. Sci. 2018, 8, 2694 3 of 15

anAppl. established Sci. 2018, 8, x limits FOR PEER and REVIEW operating conditions, and (3) to confirm the selection of the appropriate3 of 15 approach based on the fouling rates of the membrane filtration system. 2. Materials and Methods 2. Materials and Methods 2.1. Current Wastewater Treatment 2.1. Current Wastewater Treatment Wastewater from the dairy plant was considered influent to the wastewater treatment process Wastewater from the dairy plant was considered influent to the wastewater treatment process and and stored the 946 m3 equilibrium tank (EQ). This tank was used as a buffer tank and also to homogenize stored the 946 m3 equilibrium tank (EQ). This tank was used as a buffer tank and also to homogenize the different kinds of wastewater coming at different times. From the EQ tank, this homogenized colloidal the different kinds of wastewater coming at different times. From the EQ tank, this homogenized liquid was added with coagulants and flocculants, before it was sent to the DAF. The pH of the feed colloidal liquid was added with coagulants and flocculants, before it was sent to the DAF. The pH of the from the EQ tank was brought down to 4.5 before the addition of coagulant, since the coagulant feed from the EQ tank was brought down to 4.5 before the addition of coagulant, since the coagulant works best at this pH. Then, sodium hydroxide was used to bring the pH back up to 7, before the works best at this pH. Then, sodium hydroxide was used to bring the pH back up to 7, before the addition of flocculants, which works best at this pH. After , air bubbles from a compressor addition of flocculants, which works best at this pH. After flocculation, air bubbles from a compressor were added to this mixture; it was then introduced to the bottom of the DAF. The suspended particles, were added to this mixture; it was then introduced to the bottom of the DAF. The suspended particles, fats, oils and greases will rise up to the top of the DAF tank by adhering to the air bubbles. Then, this fats, oils and greases will rise up to the top of the DAF tank by adhering to the air bubbles. Then, top layer was skimmed off by mechanical skimmers, and set aside as sludge. The clean effluent comes this top layer was skimmed off by mechanical skimmers, and set aside as sludge. The clean effluent out at the other side of the DAF and was sent to the municipality. Figure 1 depicts the flow chart of comes out at the other side of the DAF and was sent to the municipality. Figure1 depicts the flow the current wastewater treatment. Samples were collected at two places: after the EQ tank, and from chart of the current wastewater treatment. Samples were collected at two places: after the EQ tank, the treated effluent. and from the treated effluent.

Influent

Equilibrium tank

Pump

H2SO4 NaOH Sludge DAF Effluent Coagulant Flocculant

Air

Compressor

Figure 1. Process flow of current wastewater treatment method. Figure 1. Process flow of current wastewater treatment method. 2.2. Membranes 2.2. Membranes Tubular membranes of 0.3048 m length and 0.0127 m diameter and a surface area of 0.012 m2 were usedTubular for thismembranes study. In orderof 0.3048 to determine m length whichand 0.0127 membrane m diameter was able and to dischargea surface area below of the 0.012 limit, m2 sixwere different used for pore this sizes study. of this In order membrane to determine size were which used. membrane The whole was spectra able ofto membrane discharge below filtration the werelimit, covered six different by the choice pore sizes of membranes, of this membrane starting from size micro were filtration used. The up whole to reverse spectra osmosis of membrane filtration. Afiltration pilot scale were membrane covered filtration by the choice skid was of used membranes, (Model BRO/BUF starting from Membrane micro filtration Specialists, up Hamilton, to reverse OH,osmosis USA) filtration. operated A by pilot a Hydra-cell scale membrane high pressure filtration piston skid pump was (Model used (Model D15EASTHFEHF, BRO/BUF Membrane Wanner Engineering,Specialists, Hamilton, Minneapolis, OH MN,, USA USA)) operated with variable by a Hydra frequency-cell drive. high pressure The characteristics piston pump of each (Model of theseD15EASTHFEHF, six membranes Wanner are described Engineering, in Table Minneapolis,1. MN, USA) with variable frequency drive. The characteristics of each of these six membranes are described in Table 1.

Appl. Sci. 2018, 8, 2694 4 of 15

Table 1. Membrane characteristics [20].

Membrane Retention Membrane Maximum Maximum pH Range Commercial Name Characteristic (kDa) Material Pressure (bar) Temperature (◦C) FP200 200 PVDF * 1.5–12 10 80 FPA03 20 PVDF * 1.5–10.5 7 60 PU608 8 Polysulphone 1.5–12 30 80 ES404 4 Polyethersulphone 1.5–12 30 80 AFC30 0.083 Polyamide film 1.5–9.5 60 60 AFC99 ** 0.058 Polyamide film 1.5–12 64 80 * Polyvinylidene fluoride, ** .

Membranes selected for this study are made up of different materials, but made by the same manufacturer (PCI membranes, Fareham, England, UK). Depending upon the pore size and the material of which the membrane were made, their operating conditions vary. Retention characteristics define the pore size. For example, a retention characteristic of 200 kDa means that any substance that is equal to or greater than this molecular weight (or equivalent hydrodynamic radii) would not pass through the membrane pores, and would be retained on the retentate side. All membranes were cleaned by the procedure defined by PCI membranes before use. The general five steps of clean-in-place were performed—pre-rinse, alkaline wash, rinse, acid wash and final rinse. The concentrations, cleaner type, and the temperature of cleaning are defined in Table2.

Table 2. Membrane cleaning characteristics [20].

Membrane Type Cleaning chemistry Acid Temperature (◦C) AFC99 0.25% sodium hydroxide 0.3% nitric acid 50 AFC30 0.5% enzyme 0.3% nitric acid 45 Others 1% chlorinated alkaline detergent 0.3% nitric acid 50

2.3. Filtration Experiment The six different membranes cannot operate at the same pressure. For example, at low pressures, the smaller pore sized membranes like 0.083 kDa and 0.058 kDa pore sized membranes, will not flux. In this case, the osmotic pressure would be higher than the driving force. And in the other end, if higher pressures were used, bigger pore sized membranes, like the micro filters, would not be able to handle this high pressure, and would therefore crush. A membrane housing with six slots was used for this study, as seen in Figure2. Filters of each type were filled in these slots one at a time. For each two sets of filters, same feed was used (ran in parallel). Figures3 and4 show the membrane filtration experiment and post-DAF membrane filtration experiment respectively. During the membrane filtration experiment, the feed from the equilibrium tank went into a 189 L tank, which was then pumped through the membranes in the membrane module. A variable frequency controlled piston pump was used to pump the feed from the tank. The pressure side of the pump had a baffler, temperature and pressure sensors, and a pressure relief valve. The baffler was used to reduce any pulsations caused by the positive displacement pump. The various operating pressures chosen for each set of membranes are listed in Table3. The permeate comes out of the circumference of the membranes and was collected and sent down to the municipality. Appl. Sci. 2018, 8, 2694 5 of 15 Appl.Appl. Sci. Sci. 2018 2018, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 55 of of 15 15

FigureFigure 2 2. .Membrane Membrane housing housing with with six six slots slots for for 1 1 foot, foot, ½ 1½ inch inch diameter diameter tubular tubular membranes. membranes. Figure 2. Membrane housing with six slots for 1 foot, 2 inch diameter tubular membranes.

Figure 3. Process flow for membrane filtration experiment. a—Baffler; b—Temperature sensor; Figure 3. Process flow for membrane filtration experiment. a—Baffler; b—Temperature sensor; c— Figurec—Pressure 3. Process sensor. flow for membrane filtration experiment. a—Baffler; b—Temperature sensor; c— PressurePressure sensor. sensor. Appl. Sci. 2018, 8, 2694 6 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 15

FigureFigure 4. 4Process. Process flow flow for for post-DAF post-DAF membrane membrane filtration filtration experiment. a a—Baffler;—Baffler; b— b—TemperatureTemperature sensor;sensor c—Pressure; c—Pressure sensor. sensor.

TheThe retentate retentate was was sent sent back back intointo thethe feedfeed tank to to concentrate concentrate it itin in order order toto get get more more recovery. recovery. OnceOnce the the feed feed was was too too concentrated,concentrated, it it would would be be drained drained to tothe the municipality municipality during during down down time of time the of thefactory. factory. A Ann automated automated back back-pressure-pressure valve valve was was used used to to achiev achievee very very high high pressures pressures through through feedbackfeedback control. control For. For all all three three experiments experiments of of membrane membrane filtration filtration experiment,experiment, the pump was was run run at at a volumetrica volumetric flow flow rate rate of 1135.62 of 1135.62 L/h. L/h. DuringDuring the the post-DAF post-DAF membrane membrane filtrationfiltration experiment,experiment, as as seen seen in in Figure Figure 44, ,the the feed feed from from the the equilibriumequilibrium tank tank was was added added with with coagulants coagulants andand flocculantsflocculants with with pH pH correction correction,, and and then then pumped pumped intointo the the bottom bottom of of the the DAFDAF together with with air air bubbles. bubbles. The The sludge sludge was wasskimmed skimmed off from off the from top the and top andthe the clear clear effluent effluent was was then then sentsent into intoa tank a tankto be tofed be to fed the tomembrane the membrane filtration filtration unit. A piston unit.A pump piston pumpwas was used used to flow to flow this thistreated treated water water through through the membranes the membranes and the and permeate the permeate was sent was to sent the to themunicipality. municipality. The The retentate retentate was was sent sent back back to to the the start start of of treatment, treatment, just just before before the the addition addition of of coagulants.coagulants. This This was was basically basically two two treatments—DAF treatments—DAF and membrane filtration filtration in in series. series.

Table 3. Operation pressures of the various membranes used. Table 3. Operation pressures of the various membranes used. Retention Operating MembraneMembrane Retention characteristic (kDa) Operating Pressure (bar) characteristic (kDa) Pressure (bar) FP200FP200 200200 6.89 6.89 FPA03 20 6.89 FPA03 20 6.89 PU608 8 24.13 ES404PU608 48 24.13 24.13 AFC30ES404 0.0834 24.13 58.61 AFC99AFC30 0.0580.083 58.61 58.61 AFC99 0.058 58.61 2.4. Analytical Methods 2.4. Analytical Methods COD was measured spectrophotometrically at 620 nm using a BioTek Epoch plate reader (BioTec COD was measured spectrophotometrically at 620 nm using a BioTek Epoch plate reader (BioTec Inc, Winooksi, VT, USA). Combined with CHEMetrics COD vials (CHEMetrics Inc., Midland, VA, USA) Inc, Winooksi, VT, USA). Combined with CHEMetrics COD vials (CHEMetrics Inc., Midland, VA, and Bioscience Digital COD reactor (BioScience Inc., Allentown, PA, USA). Potassium dichromate was USA) and Bioscience Digital COD reactor (BioScience Inc., Allentown, PA, USA). Potassium the oxidizing agent used to oxidize the organic material in the wastewater in the presence of silver dichromate was the oxidizing agent used to oxidize the organic material in the wastewater◦ in the catalystpresence and of mercury silver catalyst to withstand and mercury chlorine to withstand interferences. chlorine The interferences. vials were heated The via tols 150 wereC heated for 2 h to and the concentration of chromic ions was determined using a spectrophotometer. Fats, oil and grease Appl. Sci. 2018, 8, 2694 7 of 15

(FOG) concentration was determined using hexane extraction. Total solids (TS), and total suspended (TSS) and dissolved solids (TDS) were measured by gravimetric analysis, heating to 110 ◦C using an oven and a 45 µm filter. Protein analysis was done by the BCA (bicinchoninic acid assay) method. Statistical analyses for comparisons of the different treatment methods and different membrane sizes were done using one-way ANOVA and Tukey test using JMP Pro 12 software. Significance of p < 0.05 was used for all comparisons.

2.5. Operating Conditions

2.5.1. Membrane Selection Trials were carried out in two different experiments for membrane selection; (1) exclusive membrane filtration (Figure3) and (2) post-DAF membrane filtration (Figure4). In membrane filtration, three of 200 kDa cut-off membrane and 20 kDa cut-off membrane were put into the six slots of the membrane housing. The membranes were cleaned by the CIP procedure first, before testing. The piston pump was operated at 20Hz, which is equivalent to a flow rate of 1135.62 L/h. The pressure was set to 6.89 bar and samples of the feed and the permeate out of each membrane were collected and analyzed for the various water quality attributes. Volumetric flow rate tests were carried out for each membrane to determine the permeate flow rate. The membranes were discarded and then the next two were put into the housing and the whole procedure was repeated, but at different pressures. Table3 states the pressure at which these membranes were operated. After all six different membranes were tested, the whole experiment was repeated two more times for replication. The same methodology was followed for post-DAF membrane filtration, with the feed in to the membrane filrtation test being the DAF effluent water. In order to calculate the separation efficiency of the membrane, the reduction percentage was calculated using: ! Cp Reduction in COD,% = 1 − 100% (1) Cf where Cp and Cf are the concentration of the permeate and the feed, respectively.

2.5.2. Extended Fouling Study In this study, the retentate and the permeate was sent down the drain. Bucket flow rate tests were carried out on permeate of the membrane to determine flux, and thus, the level of fouling. Permeate fluxes were reported in L/(m2h)) and were calculated by:

V Permeate flux = (2) At where V is the permeate volume in L, A is membrane surface area in m2 and t is time in h. Extended fouling study was done only on the selected membranes. In this study, the temperature was monitored in order to eliminate the influence of temperature on flux. All fluxes J(T) measured at temperature T were corrected to J(40), measured at 40 ◦C by [21]:

J(40) = J(T)1.02540−T (3)

As the feed from the dairy plant was shut down, the experiment was stopped. In order to determine the fouling rate, the data from this study was modelled as:

J = a + b exp(−ct) (4) Appl. Sci. 2018, 8, 2694 8 of 15 where, ’J’ is the flux (L/(m2h)) ‘a’ is the asymptote (L/(m2h)), also called as the critical flux. ‘b’ is the scale (L/(m2h)) which defines the curvature of the model, ‘c’ is the fouling rate (1/h) and ‘t’ is the time (h). This model is called the exponential fouling model.

3. Results and Discussion

3.1. Feed Source Characterization The feed to the wastewater treatment plant was characterized based on pH, free chlorine, COD, protein, FOG, TS and TSS concentrations. It is important to know what kind of wastewater is being dealt with before any experiments are done, as this gives an idea on how it may affect the different membranes being studied. Concentrations of these various materials in the wastewater are depicted in Table4. A total of 12 samples of the feed were collected on four different days and analyzed. The heterogeneity in the feed is seen in terms of standard deviation, and it also depends on the moment at which the samples are taken. The wastewater predominantly contains CIP solutions, which is why the pH is 11.32 ± 0.55, i.e., on the alkaline side of the scale.

Table 4. Feed source characterization.

Parameter Concentration ± SD pH 11.32 ± 0.55 Free Cl 5 ± 1 ppm COD 8201 ± 3010 ppm Protein 1970 ± 359 ppm FOG 98 ± 113 ppm TS 5857 ± 1261 ppm TSS 2 ± 1 ppm

The presence of free chlorine in the wastewater is due to the usage of chlorinated caustic cleaners, chlorine sanitizers, and salt use during product manufacturing. Levels of 5 ± 1 ppm of free chlorine are very low, and won’t affect the pores of an RO membrane. Thus, no pretreatment is necessary while using RO membrane. Dairy products finding their way down the drain from various pathways are the primary origin for COD, protein, FOG and solids in the wastewater. From Table4, it is also seen that total suspended solids constitute only a very small part of the total solids. This means that the wastewater contains a huge amount of dissolved solids due to dumping of acid whey down the drain. Thus, the feed source characterization gives an overall idea on what is being dealt with.

3.2. Chemical Oxygen Demand

3.2.1. Comparison of Input and Output COD in Various Treatments The input and output COD of the DAF are compared with the input and output CODs of different membranes in each experiment, as seen in Figure5. The input and output COD are referred to the feed and permeate/effluent COD respectively. The feed to the DAF and each set of the membranes are the same in part A of Figure5, but the values are different, since each set of these experiments was started with new feed. The COD limit of 1200 ppm is seen as the dotted line across the two parts of the figure. The DAF discharged to the municipality at 1455 ± 504 ppm, which is over the limit at certain times, thus creating the need for a specific treatment to consistently discharge below the limit. There was no significant difference between the output COD of the DAF and the permeate CODs of the 200 kDa and 20 kDa cut-off membranes in both experiments. This can be seen more evidently in Figure6. However, the DAF output COD was not constantly below the limit. The permeate COD of these two membranes in both experiments was below the limit (small standard deviation), which means that these membranes are consistently discharging below the limit, independent of the input feed COD. This is very important in industries to avoid surcharges. In Figure5 part B, the output of the DAF is Appl. Sci. 2018, 8, 2694 9 of 15 same as the input to the membranes in this experiment. It is observed that the COD of the permeate reduces with decrease in pore size. This is because as the pore size reduces, less organic material can go throughAppl. Sci. these2018, 8 smaller, x FOR PEER membrane REVIEW pores, and thus, the COD decreases. In both A and B, there were9 noof 15 significant differences between the permeate CODs of the various set of membranes (200 and 20 kDa, 8various and 4 kDa, set 0.083of membranes kDa and 0.058(200 kDa).and 20 Thus, kDa, another 8 and 4 parameter kDa, 0.083 is kDa required and 0.058 to select kDa). a membrane Thus, another at a certainparameter pressure. is required to select a membrane at a certain pressure.

Figure 5. Chemical Oxygen Demand (COD) of the feed and permeate stream of different membranes in (A) membrane filtration and (B) post-DAF membrane filtration experiment.

If the limitFigure on 5 COD. Chemical is 1200 Oxygen ppm, Demand then a 200(COD kDa) of or the a feed 20 kDa and cut-offpermeate membrane stream of woulddifferent suffice for this purpose.membranes Energy consumptionin (A) membrane is importantfiltration and in (B membrane) post-DAF selection.membrane Iffiltration two different experiment. membranes are able to discharge below the limit, then the larger pore sized membrane should be selected, since it If the limit on COD is 1200 ppm, then a 200 kDa or a 20 kDa cut-off membrane would suffice for could be run at lower pressure/speed. If the limit is lower, then smaller pore sized membranes could this purpose. Energy consumption is important in membrane selection. If two different membranes be chosen. A 0.058 kDa cut-off membrane provides the lowest permeate COD when compared to all are able to discharge below the limit, then the larger pore sized membrane should be selected, since other membranes, and thus, could be used for reclamation purposes. it could be run at lower pressure/speed. If the limit is lower, then smaller pore sized membranes 3.2.2.could Reduction be chosen. in A COD 0.058 kDa cut-off membrane provides the lowest permeate COD when compared to all other membranes, and thus, could be used for reclamation purposes. DAF provides a 75.15 ± 3.95 % reduction in COD, and this is statistically the same when compared to3.2.2. 200 kDaReduction and 20 in kDa COD pore sized membranes in membrane filtration experiments. The membranes have a higher standard deviation than the DAF because the feed COD changes with time, but the permeateDAF COD provides – in our a results75.15 ± – remained3.95 % reduction constant in membrane COD, and filtration. this is statistically It was found the that same 200 when kDa andcompared 20 kDa to pore 200 sizedkDa and membranes 20 kDa pore in membrane sized membranes filtration in experiment membraneare filtration no more experiment efficients than. The themembranes DAF, but have they a are higher more standard consistent deviation than the than DAF. the From DAF Figurebecause6, the it is feed observed COD changes that the with percent time, CODbut the reduction permeate increases COD –with in our a decreasedresults – remain pore size.ed constant Among in all membrane different treatment filtration. methods, It was found the ROthat membrane200 kDa and in post-DAF20 kDa pore membrane sized membranes filtration providedin membrane the highest filtration percent experiment reduction are (98.83no more± 0.21efficient %). Anotherthan the important DAF, but observation they are more is that consistent the difference than in the percent DAF. reduction From Figure between 6, it theis observed two experiments that the forpercent a particular COD reduction membrane increases reduces with as the a decrease pore sized ofpore the size. membrane Among decreases.all different In treatment cases where methods, high percentthe RO reductionsmembrane inin CODpost-DAF are required membrane and filtration a smaller provide pore sizedd the membranehighest percent like areduction 0.083 kDa (98.83 or a ± 0.0580.21 kDa%). Another cut-offmembrane important wasobservation chosen, is it that is better the difference to choose in the percent membrane reduction filtration between experiment the two insteadexperiments of the for post-DAF a particular membrane membrane filtration reduces experiment, as the pore since size the of percentthe membrane COD reductions decreases. areIn cases not significantlywhere high differentpercent reductions between the in twoCOD experiments are required in and those a smaller two membranes, pore sized andmembrane it is economically like a 0.083 adventageouskDa or a 0.058 to runkDa one cut- treatmentoff membran methode was instead chosen, of twoit is in better series. to Therefore, choose the the membrane larger pore filtration sized membranes—200experiment instead kDa, of20 kDa, the post 8 kDa-DAF and 4 membrane kDa—are selected filtration for experiment, fouling studies, since since the they percent operate COD at lowerreductions pressures, are not which significantly means lower different costs between than the the smaller two experiments pore sized membranes—0.083in those two membrane kDas and, and it is economically adventageous to run one treatment method instead of two in series. Therefore, the larger pore sized membranes—200 kDa, 20 kDa, 8 kDa and 4 kDa—are selected for fouling studies, since they operate at lower pressures, which means lower costs than the smaller pore sized membranes—0.083 kDa and 0.058 kDa. Percent reductions in COD were calculated using Equation (1). Fouling rate is the most important parameter in the model as it depicts the rate at which the membrane fouls, and determines the frequency of cleaning, and henceforth, replacement. Appl. Sci. 2018, 8, 2694 10 of 15

0.058 kDa. Percent reductions in COD were calculated using Equation (1). Fouling rate is the most important parameter in the model as it depicts the rate at which the membrane fouls, and determines Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 15 the frequency of cleaning, and henceforth, replacement. Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 15 DAF Membrane Filtration Post-DAF membrane filtration 100 DAF Membrane Filtration Post-DAF membrane filtration 90100 90 80 80 70 70 60 60 50 50 40 40

30 30 Reduction COD in (%) Reduction COD in (%) 20 20 10 10 0 0 DAF 200 kDa 20 kDa 8 kDa 4 kDa 0.083 kDa 0.058 kDa DAF 200 kDa 20 kDa 8 kDa 4 kDa 0.083 kDa 0.058 kDa Membrane retention characteristic Membrane retention characteristic

FigureFigure 6. 6Percent. Percent reduction reduction inin COD across across various various treatment treatment methods. methods. Figure 6. Percent reduction in COD across various treatment methods. 3.2.3.3.2.3. Permeate Permeate Flux Flux 3.2.3. Permeate Flux The permeateThe permeate fluxes fluxes of different of different membranes membranes in both in experimentsboth experiments are shown are shown in Figure in Figure7 (calculated 7 using(calculatedThe Equation permeate using 2). Flux Equation fluxes is a functionof 2). different Flux is of a temperature function membranes of temperature and in pressure.both and experiments Onlypressure. the Only membranes are shownthe membranes which in Figure were 7 operated(calculatedwhich at were using the operated same Equation pressure at the 2). same Flux can p beressureis a compared. function can be compared.of Within temperature each Within set and each of membranes,pressure. set of membranes Only it isthe, seenit membranesis seen that the that the smaller pore sized membrane has a lower flux when compared to the larger pore sized smallerwhich were pore operated sized membrane at the same has p aressure lower can flux be when compared. compared Within to the each larger set of pore membranes sized membrane., it is seen membrane. This is because as the pore size reduces, it would be difficult for particles to move across Thisthat is the because smaller as thepore pore sized size membrane reduces, it wouldhas a lower be difficult flux whenfor particles compared to move to the across larger the membrane.pore sized the membrane. And also, in each membrane, the post-DAF membrane filtration flux is higher than Andmembrane.the also, membrane in This each is membrane,filtration because flux. as the This pore post-DAF is because size reduces membrane the feed, it towould filtrationthe membranes be difficult flux is in higherfor membrane particles than filtration theto move membrane is a cross filtrationthe membrane.more flux. concentrated, This And is becausealso, leading in each the to feedhigher membrane, to osmotic the membranes the pressure post-DAF inwhe membrane nmembrane compared filtration tofiltration the feed is moreflux in postis concentrated, higher-DAF than leadingthe membranemembrane to higher filtration. filtration osmotic flux. pressure This whenis because compared the feed to the to feedthe membranes in post-DAF in membrane membrane filtration. filtration is more concentrated, leading to higher osmotic pressure when compared to the feed in post-DAF membrane filtration.

2 Figure 7.FigurePermeate 7. Permeate fluxes (L/(mfluxes (L/h))(m of2h) different) of different membranes membranes in bothin both experiments—membrane experiments—membrane filtration and post-DAF membrane filtration.filtration and post-DAF membrane filtration.

3.3. Extended Fouling Study Figure 7. Permeate fluxes (L/(m2h)) of different membranes in both experiments—membrane filtration and post-DAF membrane filtration.

3.3. Extended Fouling Study Appl. Sci. 2018, 8, 2694 11 of 15

3.3.Appl. ExtendedSci. 2018, 8, Foulingx FOR PEER Study REVIEW 11 of 15 The membranes chosen for extended fouling study were the larger pore sized 200 kDa, 20 kDa, The membranes chosen for extended fouling study were the larger pore sized 200 kDa, 20 kDa, 8 kDaAppl. Sci. and 2018 4, 8 kDa, x FOR cut-off PEER REVIEW membranes. The permeate flux reduces with time as the membrane11 of 15 fouls 8 kDa and 4 kDa cut-off membranes. The permeate flux reduces with time as the membrane fouls and asymptotes or equilibrates to a certain flux called the critical flux [22]. Figures8 and9 depict the and asymptotesThe membranes or equilibrates chosen for toextended a certain fouling flux studycalled were the criticathe largerl flux pore [22 sized]. Figure 200 kDa,8 and 20 FigurekDa, 9 fouling of 200 kDa and 20 kDa cut-off membranes at 100 psi and 8 kDa and 4 kDa cut-off membranes depict8 kDa the and fouling 4 kDa ofcut 200-off kDa membranes. and 20 kDa The cutpermeate-off membranes flux reduces at 100with psi time and as 8 thekDa membrane and 4 kDa fouls cut -off at 350 psi, respectively. The flux reduces exponentially with time for first few minutes, and then it membranesand asymptotes at 350 or psi, equilibrates respectively. to a Thecertain flux flux reduces called exponentially the critical flux with [22 ].time Figure for first8 and few Figure minutes 9 , startsanddepict then to the equilibrate it startsfouling to of toequilibrate 200 a criticalkDa and to flux 20a criticalkDa with cut increase flux-off membraneswith in increase time. at Therefore, 100in time.psi and Therefore the 8 rawkDa dataand, the 4 was kDaraw modelled cutdata-off was with exponentialmodelledmembranes with foulingat exponential 350 psi, models, respectively. fouling and themodels The parameters flux, and reduces the ofparameters exponentially the model of werewiththe model time determined for were first determined few from minutes least from, sqaures, andleastand aresqaures then shown it ,starts and in areto Table equilibrate shown5. It in is Table seento a critical that5. It is the flux seen 200 with that kDa increase the cut-off 200 inkDa membranetime. cut Therefore-off membrane has, the raw highest has data the criticalhighestwas flux (asymptote)criticalmodelled flux with(asymptote) of 95.4exponential± 0.84of 95.4 fouling L/(m ± 0.84 2modelsh) L/ which(m, 2andh) iswhich the related parameters is related to the of to biggestthe the model biggest pore were pore size. determined size. The The 20 from kDa20 kDa cut-off membranecutleast-off sqauresmembrane has, and the has are lowest theshown lowest critical in Table critical flux 5. Itflux of is 62.01seen of 62.01 that± 2.48the± 2.48 200 L/(m L/ kDa(m2 2h)cuth) and -off membranefouling fouling rate rate has of ofthe 0.11 0.11 highest ± 0.02± 0.02 h−1 h−1 amongcritical the flux four four (asymptote) membranes membranes of 95.4studied. studied. ± 0.84 Lower L/ Lower(m2 h)fouling which fouling rate is ratesrelateds relate relate to to the longer to biggest longer lifetime pore lifetimes size.s of theThe of membrane the20 kDa membrane; ; 2 −1 thus,thuscut,- offthe membrane 20 20 kDa kDa cut cut-off has-off the membrane membrane lowest critical is isdesirable, desirable, flux of 62.01 even even ± though 2.48 though L/(m its h) itscritical and critical fouling flux flux is ratelower. is lower.of 0.11 ± 0.02 h among the four membranes studied. Lower fouling rates relate to longer lifetimes of the membrane; thus, the 20 kDa cut-off membrane is desirable, even though its critical flux is lower. 140 200 kDa 20 kDa Model - 200 kDa Model - 20kDa

120140 h))

2 200 kDa 20 kDa Model - 200 kDa Model - 20kDa

100120

h)) 2 80100

6080

4060

Permeate Flux (L/(m 2040

Permeate Flux (L/(m 20 0 00 5 10 15 20 Time (h) 0 5 10 15 20 Time (h) Figure 8 8.. ExponentialExponential fouling fouling model model on 200 on kDa 200 and kDa 20 kDa and in 20 membrane kDa in membrane filtration experiment filtration at experiment 6.89 bar. at 6.89Figure bar. 8. Exponential fouling model on 200 kDa and 20 kDa in membrane filtration experiment at 6.89 bar. 160 160 8 kDa 4 kDa Model - 8 kDa Model - 4 kDa 140 8 kDa 4 kDa Model - 8 kDa Model - 4 kDa

h)) 140 2

120h)) 2 120 100 100 80 80 60 60

4040 Permeate Flux Flux Permeate(L/(m Permeate Flux Flux Permeate(L/(m 2020 0 0 00 22 4 66 8 8 1010 12 12 14 14 Time (h) Time (h) FigureFigure 99. .9 Exponential. Exponential fouling fouling fouling model model on 8 on kDakDa 8 kDa and and and4 4 kDa kDa 4 in kDa in membrane membrane in membrane filtration filtration filtration experiment experiment experiment at 24.13 at 24.13 bar at .bar 24.13. bar.

TableTableTable 5 5.5. .Exponential ExponentialExponential fouling fouling modelmodel model parameters parameters parameters (Mean (Mean (Mean ± standard± standard± standard error) error). error)..

Membrane AsymptoteAsymptote (L/(m2h)) Scale ScaleScale (L/(m 2Foulingh))Fouling Rate FoulingRate Rate (1/h) MembraneMembrane (L/(m22h)) (L/(m2h)2 ) (1/h) 200 kDa 95.40 ±(L/0.84(m h)) (L/(m 31.59h)±) 2.02(1/h) 0.50 ± 0.06 200 kDa 95.40 ± 0.84 31.59 ± 2.02 0.50 ± 0.06 20 kDa 200 kDa 62.0195.40± 2.48 ± 0.84 31.59 30.09 ± 2.02± 2.180.50 ± 0.06 0.11 ± 0.02 8 kDa 2020 kDakDa 66.4362.01± 1.00 ±± 2.482.48 30.0930.09 56.54± ±2.18 2.18± 1.940.110.11 ± 0.02± 0.02 0.56 ± 0.05 4 kDa 88 kDa kDa 71.3766.43± 0.62 ±± 1.001.00 56.5456.54 64.34± ±1.94 1.94± 0.880.560.56 ± 0.05± 0.05 0.40 ± 0.01 4 kDa 71.37 ± 0.62 64.34 ± 0.88 0.40 ± 0.01 4 kDa 71.37 ± 0.62 64.34 ± 0.88 0.40 ± 0.01 Appl.Appl. Sci. Sci.2018 2018, 8,, 2694 8, x FOR PEER REVIEW 12 of1215 of 15

3.4. Solids Content 3.4. Solids Content Total solids present in the feed and output/permeate of different treatments are depicted in FigureTotal 10 solids. The presentDAF (A) in produced the feed a and 37.69% output/permeate reduction in TS of from different 3932 ppm treatments to 2424 are ppm. depicted In part inB of FigureFigure 10 .10 The, the DAF permeate (A) produced of the RO a 37.69% membrane reduction had the in lowest TS from TS 3932, 311 ppm ppm. to The 2424 feed ppm. to this In part treatment B of Figuremethod 10, theand permeate DAF are ofthe the same. RO The membrane feed to hadthe treatment the lowest method TS, 311 C ppm. post The-DAF feed membrane to this treatment filtration is methodsame as and the DAF output are theof the same. DAF. The In feedthis method, to the treatment the permeate method of Cthe post-DAF RO membrane membrane had the filtration lowest is TS, samei.e., as 145 the ppm. output This of theTS is DAF. lower In thisthan method, that of the the RO permeate membrane of the in RO treatment membrane B, hadsince the the lowest feed to TS, the i.e.,latter 145 ppm. treatment This TS is is higher. lower The than feed that of to the the RO membrane membrane filtration in treatment experiment B, since (B) the has feed solids to the that latter are treatmentgreater isthan higher. 200 kDa The, feedand the to the feed membrane to the post filtration-DAF membrane experiment filtration (B) has (C) solids has that solids are that greater are greater than 200than kDa, 8 andkDa, the but feed smaller to the than post-DAF 20 kDa. membrane So, the DAF filtration is removing (C) has the solids solids that which are greater are bigger than 8 than kDa, 20 butkDa, smaller but is than not 20 effective kDa. So, at theremoving DAF is solids removing smaller the solidsthan that. which are bigger than 20 kDa, but is not effective at removing solids smaller than that. 3.5. Protein Concentration 3.5. Protein Concentration Proteins present in the feed and output/permeate of different treatments are depicted in Figure 11Proteins. The DAF present (A) produced in the feed a 50.12% and output/permeate reduction in protein of different from 1351 treatments ppm to are625depicted ppm. In the in Figure membrane 11. Thefiltration DAF (A) experiment produced a(part 50.12% B), the reduction protein in concentration protein from in 1351 the ppmpermeate to 625 reduced ppm. In with the membranea decrease in filtrationpore size. experiment In post-DAF (part membrane B), the protein filtration concentration experiment in the(part permeate C), the reducedprotein reduction with a decrease percentage in porewas size. very In low. post-DAF This means membrane that the filtration DAF has experiment removed all (part proteins C), the bigger protein than reduction 20 kDa. percentageAdditionally, wasthe very protein low. concentration This means that in the the permeate DAF has removeddecreased all with proteins a decrease bigger in than pore 20 size kDa. in both Additionally, membrane thefiltration protein concentrationand post-DAF inmembrane the permeate filtration decreased experiments. with a decrease in pore size in both membrane filtration and post-DAF membrane filtration experiments.

Figure 10. Total solids in the feed and the output/permeate of different treatments: A—DAF; Figure 10. Total solids in the feed and the output/permeate of different treatments: A—DAF; B— B—membrane filtration; C—post-DAF membrane filtration. membrane filtration; C—post-DAF membrane filtration.

Appl.Appl. Sci. Sci.2018 2018, 8,, 2694 8, x FOR PEER REVIEW 1313 of of15 15

Figure 11. Protein concentration in the feed and the output/permeate of different treatments: A—DAF; Figure 11. Protein concentration in the feed and the output/permeate of different treatments: A— B—membrane filtration; C—post-DAF membrane filtration. DAF; B—membrane filtration; C—post-DAF membrane filtration. 4. Conclusions 4. Conclusions The application of membrane filtration to the management of waste water streams from a dairy The application of membrane filtration to the management of waste water streams from a dairy manufacturing operation has been completed. The waste water stream had a pH of 11.32 ± 0.55 and manufacturing operation has been completed. The waste water stream had a pH of 11.32 ± 0.55 and was heterogeneous in terms of COD, protein and total solids. A Dissolved Air Flotation (DAF) system was heterogeneous in terms of COD, protein and total solids. A Dissolved Air Flotation (DAF) system removed large particles, total solids and proteins. The DAF system was not effective in reducing the removed large particles, total solids and proteins. The DAF system was not effective in reducing the COD of the waste stream to below a target COD of 1200 ppm on a consistent basis. COD of the waste stream to below a target COD of 1200 ppm on a consistent basis. AllAll six six membranes membranes (200, (200, 20,20, 8, 4, 4, 0.083, 0.083, 0.058 0.058 kDa) kDa) reduced reduced the permeate the permeate COD CODto less to than less 1200 than 1200ppm. ppm. These These reductions reductions were were independent independent of the of COD the CODof the of feed the stream. feed stream. The permeate The permeate COD from COD fromthe the membranes membranes was was not not influenced influenced significantly significantly by by pressure. pressure. The The reduction reduction in in permeate permeate COD COD improvedimproved as theas the membrane membrane pore pore size size decreased. decreased. TwoTwo membranes membranes with with operating operating pressures pressures ofof 100100 psipsi (200(200 and 20 kDA kDA)) and and two two with with operating operating pressurespressures of 350of 350 psi psi (8 (8 and and 4kDa) 4kDa) were were selected selected for for extendedextended fouling studies. studies. The The permeate permeate flux flux from from thethe four four membranes membranes decreased decreased with with time time due due to to fouling,fouling, andand the decrease decrease in in flux flux was was described described by by a a three-parameterthree-parameter expression, expression, including including a fouling a fouling rate rate constant. constant. The The 20 20 kDa kDa pore pore size size membrane membrane had had the lowestthe lowest fouling fouling rate in therate extendedin the extended fouling fouling study compared study compared to the other to the three other membranes. three membranes. In addition, In theaddition, 20 kDa pore the 20 size kDa membrane pore size operatedmembrane at operated a lower pressure at a lower than pressure the 8 orthan 4 kDa the pore8 or 4 size kDa membranes. pore size membranes. Author Contributions: S.N., D.P. and D.H. designed the experiments and goals of the research study; S.N. and D.P.Author Executed Contributions: pilot trails S.N., and analyzedD.P. and D.H. data; designed S.N. wrote the experiments the paper; D.H. and goals is responsible of the research for overall study; S.N. content and of theD.P. research. Executed pilot trails and analyzed data; S.N. wrote the paper; D.H. is responsible for overall content of the Funding:research.Funding for this research was received by the Center for Innovative Food Technology (Toledo, OH, USA) Project number 4703. Funding: Funding for this research was received by the Center for Innovative Food Technology (Toledo, OH, Acknowledgments:USA) Project numberThe 4703. authors wish to acknowledge the generous donations from Dale A. Seiberling. And also would like to thank OARDC (Ohio Agricultural Research and Development Center). This work was supported by theAcknowledgments: Centre for Innovative The Food authors Technology wish to acknowledge (CIFT) in Ohio. the generous donations from Dale A. Seiberling. And also would like to thank OARDC (Ohio Agricultural Research and Development Center). This work was supported Conflicts of Interest: The authors declare no conflict of interest. by the Centre for Innovative Food Technology (CIFT) in Ohio.

Conflicts of Interest: The authors declare no conflicts of interest.

Nomenclature

Abbreviations Appl. Sci. 2018, 8, 2694 14 of 15

Nomenclature

Abbreviations EQ Equilibrium tank DAF Dissolved air floatation PVDF Polyvinylidene fluoride RO Reverse osmosis COD Chemical oxygen demand BCA Bicinchoninic acid assay CIP Clean in place FOG Fats, oils and grease TS Total solids TSS Total suspended solids kDa Kilo Dalton MWWT Municipal waste Symbols Cp Permeate concentration (ppm) Cf Feed concentration (ppm) J Permeate flux (L/(m2h)) a Asymptote (L/(m2h)) b Scale (L/(m2h)) c Fouling rate h−1

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