A Preliminary Study on the Use of Xylit As Filter Material for Domestic Wastewater Treatment

applied sciences

Article A Preliminary Study on the Use of Xylit as Filter Material for Domestic Wastewater Treatment

Marcin Spychała *, Tadeusz Nawrot † and Radosław Matz

Department of Hydraulic and Sanitary Engineering, Poznan University of Life Sciences, Pi ˛atkowska94A, 60-649 Pozna´n,Poland; [email protected] (T.N.); [email protected] (R.M.) * Correspondence: [email protected] † Co-author Tadeusz Nawrot passed away before publication.

Featured Application: The study indicated xylit as a material highly effective in wastewater quality indicators removal. The material is suitable for simple construction (low cost) on-site wastewater trickling ﬁlters.

Abstract: The aim of the study was to verify two morphological forms (“angel hair” and “scraps”) of xylit as a trickling ﬁlter material. The study was carried out on two types of polluted media: septic tank efﬂuent (STE) and seminatural greywater (GW). The basic wastewater quality indicators,

namely, chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total suspended solids (TSS), ammonium nitrogen (NNH4), and total phosphorus (Ptot) were used as the indicators of treatment efficiency. Filtering columns filled with the investigated material acted as conventional 3 3 trickling filters at a hydraulic load of 376–472 cm /d during the preliminary stage, 198–245 cm /d during stage I, and 184–223 cm3/d during stage II. The removal efficiency of the two morphological forms of xylit did not differ significantly. The average efficiencies of treatment were as follows: for Citation: Spychała, M.; Nawrot, T.; Matz, R. A Preliminary Study on the COD, over 70, 80, and 85% for preliminary stage, stage I and stage II, respectively; for BOD5, 77–79% Use of Xylit as Filter Material for (preliminary stage); for TSS, 42% and 70% during the preliminary stage, and 88, 91, and 65% during Domestic Wastewater Treatment. stage I; for NNH4, 97–99% for stage I and 36–49% for stage II; for Ptot, 51–54% for stage I and 52–56% Appl. Sci. 2021, 11, 5281. https:// for stage II. The study demonstrated that xylit was a material highly effective in wastewater quality doi.org/10.3390/app11115281 indicators removal, even during the initial period of its use.

Academic Editors: Ioanna Vasiliadou, Keywords: greywater; nutrient; organic compound; wastewater; xylit Noori MD Tabish and Athanasia Tekerlekopoulou

Received: 29 April 2021 1. Introduction Accepted: 31 May 2021 Published: 7 June 2021 Individual wastewater systems are common in many regions of Europe and some parts of the world (USA), due to lack of economic justiﬁcation or sewerage systems; its

Publisher’s Note: MDPI stays neutral building needs will develop. with regard to jurisdictional claims in Septic tanks are in general devices used for preliminary wastewater treatment; how- published maps and institutional affil- ever, they are prone to periodically increased concentrations of suspended solids in effluent, iations. which may accelerate clogging processes. Clogging, it should be noted, can occur even after a few years [1]. In view of the disadvantages of septic tanks and aiming to prevent outflow from small wastewater treatment facilities from instability and exceeding recommended or required limits, there is a need to look for systems of better treatment, including nutrients, which Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. can ensure longer life of existing soil infiltration systems. Some interesting and promising This article is an open access article solutions are referred to in the literature. They utilise, e.g., a geotextile fabric filter [2], a distributed under the terms and modified up-flow septic tank, followed by an anaerobic baffled reactor [3] and a gradual conditions of the Creative Commons chamber using a gravel bed as an effluent filter [4]. Attribution (CC BY) license (https:// The authors are therefore of the opinion that there is a need to look for trickling filter creativecommons.org/licenses/by/ filling materials that are cheap, highly effective in removing contaminants, and where 4.0/). possible, ones that have a low (negative) impact on the environment (carbon footprint).

Appl. Sci. 2021, 11, 5281. https://doi.org/10.3390/app11115281 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 5281 2 of 17

Such systems should be as cheap and simple as possible, especially in less developed or developing countries. The traditional trickling filter technology preceded by a septic tank or preliminary settler still seems to be a promising solution because of its simplicity and resistance to load and hydraulic overloading. In contrast, plastic carriers being used for trickling filter filling are relatively expensive and are often associated with negative side effects on the environment during their production, such as energy consumption and carbon footprint. Xylit (usually referred to as xyloid lignite) is a waste material (by-product) obtained from the mining of lignite (brown coal). Xylit is a type of lignite, sometimes referred to as fossil wood. The use of the term is justifiable because xylit frequently has an appearance similar to wood; some properties (and visual or physical features) of “wood” are altered [5]. The structure of carbonised wood fibres can be observed in this material of density around 400 kg/m3. It is not used as fuel for heat generation due to very low heat content (even in a dried state) and due to the obstruction of the technological process of construction ceramics production [6]. Xylit is not well recognised as a filtering material for wastewater or greywater treatment and only a few studies have been carried out on this usage [5,7]; however, it is used as a biocarrier in some decentralised wastewater systems [8]. One of the most detailed research was conducted by Zhang [9]. This author highlighted several interesting and important features of xylit that can prove useful in polluted water treatment, such as (1) particle size of 20 mm, (2) solvent-accessible area (surface area approx. 2.5 m2/g), (3) pore volume of 0.01 cm3/g, (4) average pore size of about 16–17 nm, (5) overall negatively charged surfaces and molecular weight, and (6) positively correlated removal of micropollutants by xylit (with both apolar and polar surface area ratio, and pH value favouring bacteria growth). These all can be considered as features also useful for dissolved organic compound removal. For the purposes of the research experiment, xylit was crushed and sieved to obtain the relevant particle size of 2–4 mm [10]. Xylit proved to be a highly effective DOC removal material, it removed 52% DOC during the screening experiment and 89% DOC in the long-term experiment [9]. In that study, only granular activated carbon was better in DOC removal efficiency (average removal efficiency above 95%). Xyloid lignite (xylit) was examined in a column experiment involving the removal of 31 selected organic micropollutants and phosphorus using several sorbents over a period of 12 weeks, and the average removal of this material was 80 ± 28% [7]. This material proved to be less effective in removing micropollutants, as compared to GAC in the long-term column experiment; however, it demonstrated a higher removal efficiency of MPS than lignite and sand [9]. Despite a smaller surface area, xylit revealed higher removal efficiency than lignite. Chemically, xylit consists mainly of carbon and oxygen, therefore is not efficient in phosphorus removal by precipitation, as shown by [9]; in his study, Ptot was poorly removed by xylit with an efficiency of 14–22%. The role of adsorption or biological processes (biofilm development) in phosphorus removal with the use of xylit was considered in the study by Zhang et al. [7]. Zhang et al. [11] observed that the hydrophobic effect played an important role in the removal of micropollutants. Vargas et al. [12] noted that the presence of surface functional groups and the surface charge affect the adsorption capacity and the micropollutant removal mechanism. Lifespan (life cycle) has not been described in the literature. The potential reuse or transformation can be through its processing to compost or its reuse after heating it to a temperature around 300 ◦C with the aim of immobilising the adsorbed organic compounds. Despite some disadvantages, such as difficulties in uniform liquid sprinkling, the advantage of xylit is the ease of its processing. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 16

Despite some disadvantages, such as difficulties in uniform liquid sprinkling, the advantage of xylit is the ease of its processing.

2. Materials and Methods 2.1. Laboratory Setup (Model) and Experiment Conditions The experimental set was located in the laboratory of the Department of Hydraulic and Sanitary Engineering, Poznań University of Life sciences. The study was carried out Appl. Sci. 2021, 11, 5281 3 of 17 at a temperature close to room temperature (17–27 °C) from 7 May 2019 until 10 July 2020. 2. Materials and Methods 2.1. Laboratory Setup (Model) and Experiment Conditions 2.2. DescriptionThe experimental of setthe was Model located inand the laboratory Its Operation of the Department of Hydraulic and Sanitary Engineering, PoznańUniversity of Life sciences. The study was carried out at a temperatureThe research close to room model temperature consisted (17–27 ◦C) from of 7filtration May 2019 until columns 10 July 2020. (tubes) which were 100 cm long and2.2. had Description an ofinternal the Model and diameter Its Operation of 4.4 cm, made of organic glass (Figure 1). The depth of fillingThe with research the model filtering consisted material of filtration columns was (tubes)80 cm. which were 100 cm long and had an internal diameter of 4.4 cm, made of organic glass (Figure1). The depth of filling with the filtering material was 80 cm.

Figure 1. Experimental set-up: (1—transparent acrylic glass tube (L = 100 cm; Din = 4.4 cm); 2—bed Figureof xylit 1. with Experimental a height of 80 cm; 3—safetyset-up: net (1—trans (mosquito net)parent with a mesh acrylic size of 1.0glass× 1.0 tube mm; (L = 100 cm; Din = 4.4 cm); 2—bed 4—fountain pump sprinkler located centrally in relation to the pipe with a 5.0 cm sponge above the of xylitsurface with of the ﬁlling; a height 5—wastewater of 80 tank cm; with 3—safety a capacity of 30net dm 3(mos; 6—pump;quito 7—collecting net) with and a mesh size of 1.0 × 1.0 mm; 4—fountainmeasuring vessel). pump sprinkler located centrally in relation to the pipe with a 5.0 cm sponge above the surfaceDuring of the the filling; preliminary 5—wastewater stage, the study was conductedtank with on two a columnscapacity (one of column 30 dm3; 6—pump; 7—collecting and measuringwith ‘angel vessel). hair’ type material and the second column—with ‘scraps’ (material crushed into particles of several centimeters) for septic tank efﬂuent (Figure2).

During the preliminary stage, the study was conducted on two columns (one column with ‘angel hair’ type material and the second column—with ‘scraps’ (material crushed into particles of several centimeters) for septic tank effluent (Figure 2). During the first stage (I), the study was carried out on three columns (two columns with angel hair (AH) type material and one column with scraps (SCR) for septic tank effluent treatment. The significant difference between the preliminary phase and first stage (apart from the number of filtering columns) was the manner of inlet sample collection. In the first stage, the inlet wastewater (septic tank effluent) was dosed from the retention tank for all columns assuming the same concentration of pollutants. However, a few control samples showed that the concentrations at the inlets to the tubes were not exactly the same. This phenomenon could be caused by the different wastewater volumes that remained inside the pumps and dosing tubes. The mixing of the content of the retention tank was not a consideration since it could change the pollutant concentration in the direct vicinity of dosing (suction) tubes. The endings of tubes were immersed at the middle of the retention tank bottom as a bunch. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 16

Therefore, during the I and II stages, the inlet STE and greywater were collected Appl. Sci. 2021, 11, 5281 separately for every filtration column. 4 of 17 During the second stage (II), two filters filled with ‘angel hair’ were used for greywater treatment.

FigureFigure 2. Forms 2. Forms of of material material (xylit) (xylit) used: used: left:: scraps;scraps;right right: angel: angel hair. hair.

2.3. ResearchDuring Layout the first stage (I), the study was carried out on three columns (two columns with angel hair (AH) type material and one column with scraps (SCR) for septic tank effluentThe study treatment. was conducted The significant in three difference stages: between the preliminary phase and first stage • (apartPreliminary from the numberstage: septic of filtering tank columns) effluent was treatment the manner and of inlet samplemedia collection.collection In as a themixture; first stage, period: the inlet 7/16 wastewater May 2019–2/9 (septic tank July effluent) 2020; wasmeasured dosed from indicators: the retention TSS, tank COD, for all columns assuming the same concentration of pollutants. However, a few control BOD5; assumed hydraulic load: about 380–470 cm3/d; this stage was planned as a samplesstartup showed period. that Its the concentrationsadditional goal at the was inlets to to determine the tubes were the not biodegradability exactly the same. of This phenomenon could be caused by the different wastewater volumes that remained wastewater pre-treated in a septic tank, which is subject to unfavourable processes inside the pumps and dosing tubes. The mixing of the content of the retention tank was not a considerationthat often take since place it could in practice, change thesuch pollutant as extended concentration retention in thetime direct and/or vicinity increased of dosingtemperature; (suction) tubes. The endings of tubes were immersed at the middle of the retention • tankStage bottom I (experiment as a bunch. I): septic tank effluent treatment and inlet media collection sep- aratelyTherefore, for every during filter the column; I and II stages,period: the 10 inletSeptember–8 STE and greywaterNovember were 2019; collected measured separatelyindicators: for everyTSS, COD, filtration Ptot, column. NNH4; assumed hydraulic load: about 200–250 cm3/d; • StageDuring II (experiment the second stage II): semina (II), twotural filters greywater filled with (inlet ‘angel media hair’ werecollecting used forseparately greywa- for terevery treatment. filter column); period: 28 May–10 July; measured indicators: COD, Ptot, NNH4; 3 2.3.assumed Research hydraulic Layout load: about 200–250 cm /d. It Thewas studydecided was to conducted perform in experiment three stages: II, using seminatural greywater, since the experiments on greywater treatment by means of biofiltration [13] showed that grey- • Preliminary stage: septic tank effluent treatment and inlet media collection as a water can be treated biologically, and filters used previously for domestic wastewater mixture; period: 7/16 May 2019–2/9 July 2020; measured indicators: TSS, COD, BOD5; (septic assumedtank effluent) hydraulic treatm load:ent aboutcan be 380–470 subsequently cm3/d; thisused stage for greywater was planned treatment. as a startup period. Its additional goal was to determine the biodegradability of wastewater pre- 2.4. Treatedtreated Media in a, Their septic Dosing, tank, which and Collection is subject to unfavourable processes that often take Theplace septic in practice, tank effluent such as used extended for the retention study was time obtained and/or increased from an temperature;on-site wastewater treatment• Stage plant I (experiment consisting I): of septic a septic tank effluenttank and treatment soil infiltration and inlet mediasystem, collection collecting sep- and arately for every filter column; period: 10 September–8 November 2019; measured treating domestic wastewater generated by a four-person household. The wastewater indicators: TSS, COD, P ,N ; assumed hydraulic load: about 200–250 cm3/d; was collected as a septic tanktot effluent.NH4 • Stage II (experiment II): seminatural greywater (inlet media collecting separately for The proportions of components of the seminatural greywater (GW) used in the every filter column); period: 28 May–10 July; measured indicators: COD, Ptot,NNH4; study wereassumed 31%, hydraulic 62%, and load: 7% aboutof laundry, 200–250 bath cm3 /d.or shower, and washbasin, respectively. 3 The seminaturalIt was decided greywater to perform comprised experiment 12 dm II, using of natural seminatural laundry greywater, greywater since collected the fromexperiments the washing on greywater machine treatmenteffluent byafter means wash ofing biofiltration 2–5 kg of [13 clothes] showed (Ariel, that greywater Procter, and Gamble,can be treatedWarsaw, biologically, Poland) and mixed filters usedwith previouslyartificial forgreywater domestic wastewatersimulating (septic bath/shower tank greywater,effluent) treatmentprepared can using: be subsequently 3.6 g of sham usedpoo for (Head greywater & Shoulders, treatment. Procter and Gamble, Warsaw, Poland), 5.7 g of shower gel (Colgate-Palmolive, Warsaw, Poland), 0.4 g of liquid soap (Serpol-Cosmetics Ltd., Mieścisko, Poland) and 27 dm3 of tap water. The total water hardness in tap water used was 275 mg CaCO3/dm3. Appl. Sci. 2021, 11, 5281 5 of 17

2.4. Treated Media, Their Dosing, and Collection The septic tank effluent used for the study was obtained from an on-site wastewater treatment plant consisting of a septic tank and soil infiltration system, collecting and treating domestic wastewater generated by a four-person household. The wastewater was collected as a septic tank effluent. The proportions of components of the seminatural greywater (GW) used in the study were 31%, 62%, and 7% of laundry, bath or shower, and washbasin, respectively. The seminatural greywater comprised 12 dm3 of natural laundry greywater collected from the washing machine effluent after washing 2–5 kg of clothes (Ariel, Procter, and Gamble, War- saw, Poland) mixed with artificial greywater simulating bath/shower greywater, prepared using: 3.6 g of shampoo (Head & Shoulders, Procter and Gamble, Warsaw, Poland), 5.7 g of shower gel (Colgate-Palmolive, Warsaw, Poland), 0.4 g of liquid soap (Serpol-Cosmetics Ltd., Mie´scisko,Poland) and 27 dm3 of tap water. The total water hardness in tap water 3 used was 275 mg CaCO3/dm . During the preliminary stage, the filtration columns were fed with septic tank effluent. In this stage, the columns were fed with a volume of about 20 cm3 of wastewater every hour. During stage I, the filtration columns were fed with septic tank effluent; however, they were fed every two hours (with a volume of about 20 cm3). After collection, the STE was transported to the laboratory and stored in a chamber of 20 dm3 at room temperature for a few days (up to 7 days). This suggests that thanks to the extended retention time and increased temperature, there occur processes that often take place in practice (high-temperature periods, vacation users’ absence). During stage II, the filtration columns were fed with GW with a volume of about 20 cm3, every two hours. The raw GW was prepared twice a week. The collection and measurements of wastewater quality indicators in raw GW were carried out no later than one day after supplying the retention chamber with a new dose of a new portion of media; hence, the retention time of media was about 24 h. The dosing schedule for raw and treated GW sample collection was as follows: 7:00, 9:00, 11:00, and 13:00, for the dosing of GW into the filters and 8:00, 10:00, 12:00, and 14:00, for the dosing of GW into the beakers. The assumed hydraulic load during the preliminary stage was about 30–40 cm·d−1 (20–25 cm3 every hour per 15.2 cm2 of inner filtering column surface area), and during stages I and II, about 15–20 cm·d−1 (20–25 cm3 every 2 h). These filtering columns were fed with STE and GW using pumps controlled by a programmable timer. Xylit (xyloid lignite) used in this study comprised carbonised wood fibres derived from lignite. The raw lignite blocks were crushed to 10–30 mm pieces (‘scraps’) and raw lignite ‘angel hair’ was crushed to 20–40 mm pieces before being used as a filter material in the experiment. During the study (usually on a weekly basis, with a few deviations, such as holidays, a holiday break) measurements of hydraulic capacity (outflow rate) and qualitative analyzes of inflow and outflow media (STE or GW) were carried out in terms of total suspended solids (TSS), five-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), ammonium nitrogen (NNH4) and total phosphorus (Ptot). Outflows were measured twice or three times a week.

2.5. Measurements of the Wastewater Quality Indicators The samples of inlet and outlet wastewater (seminatural greywater or septic tank efﬂuent) were collected and analysed usually once a week. The following parameters were analysed: chemical oxygen demand (COD), determined by using the bichromate method (the oxidation of organic compounds using chromosulfuric acid, determination as chromate) using a Merck Spectroquant® NOVA 60A spectrophotometer; ﬁve-day biochemical oxygen demand (BOD5), performed with the use of the respirometric method (Oxitop, WTW), (ammonium nitrogen (N-NH4, the indophenol method) and total phos- ® phorus (Ptot), analysed using a Merck Spectroquant NOVA 60A spectrophotometer; total Appl. Sci. 2021, 11, 5281 6 of 17

suspended solids (TSS) using the dry weight method with filtration through paper filters. The detection of the total phosphorus and ammonium nitrogen (NNH4) concentrations was performed using spectrophotometer (Merck,Darmstadt, Germany) kits (Spectroquant kits Nos. 14752 and 14773, respectively). Determination of wastewater quality indicators, defined as dissolved organic and nutrient compounds (COD, BOD5,NNH4,Ptot, TSS) was performed for samples filtered through paper filters of 2.5 µm pore size. The values of the wastewater quality indicators were determined in accordance with the standards (for COD: PN-ISO 6060 [14], for Ptot: PN-EN ISO 6878 [15], for TSS: PN-EN 872 [16]). The pH value is not limited in Poland for on-site wastewater treatment plants treating domestic wastewater up to 5.0 m3/d and disposed into the soil being the property of the user. In view of this fact, only a few pH measurements were taken and the average values were 8.29 ± 0.03 and 8.36 ± 0.05 for inflowing and outflowing seminatural greywater, respectively.

2.6. Statistical Analysis At the beginning of the statistical procedure, it was verified whether the population of values within sets is normally distributed using the Shapiro–Wilk test. Then, a paired sample t-test of the hypothesis for the difference in means was applied [17]. The analysis of paired sample t-test of the difference between means was performed as proposed by Łomnicki [17], with final verification conducted by checking whether the critical value for the significance level 0.025 of the two-sided test was higher or lower than the calculated t statistic.

2.7. Media Properties Although the inflow wastewater (STE) was taken from the same source for the preliminary stage and stage I, the average inflow wastewater quality indicators in septic tank effluent applied into the filters during the preliminary stage and stage II were fairly different. It was probably caused by a longer time of storage in the laboratory and a higher volume of the sediments filling the septic tank (sludge). The content of basic wastewater quality indicators in the STE used was typical. The proportion of greywater content (31%, 62%, and 7% of laundry, bath and shower, and washbasin, respectively) was comparable to values reported by other authors [18,19]. The concentrations of wastewater quality indicators in seminatural greywater varied in the range typical for real greywater [20–23].

3. Results and Discussion 3.1. Outflow Rate Although the flow rates of pumps were established on approximately the same level, some differences were observed between hydraulic capacities of columns, and there oc- curred some irregularities in the flow rate of each pump. Some anomalies were related to the plugging of sprinklers, the plugging of pump inlet and outlet tubes, and rinsing after plugging. The average outflow rates during preliminary stage (7 May–27 July 2020) were as follows: 471.8 ± 49.6 cm3/d (n = 21) for column filled with AH, and 375.8 ± 38.8 cm3/d (n = 21) for column filled with SCR. The average outflow rates during stage I (10 September– 8 November) were as follows: 244.9 ± 12.9 cm3/d (n = 11), 242.4 ± 15.2 cm3/d (n = 11), and 197.8 ± 9.6 cm3/d (n = 11) for AH1, AH2, and SCR, respectively. The average outflow rates during stage II (28 May–10 July 2020) were 183.9 ± 5.1 cm3/d (n = 14) for AH1 and 223.2 ± 8.2 cm3/d (n = 14) for AH2. During the study, the effect of hydraulic load on wastewater quality indicators removal efficiency was not analysed; however, the hydraulic load was selected so that under technical conditions, it would be possible for four users (total outflow about 0.4 m3/d) to use a trickling filter of a surface area not exceeding a few square meters in top view (2–3 m2). A given reactor therefore would need to be as compact as possible to be installed Appl. Sci. 2021, 11, 5281 7 of 17

in a room, e.g., in a basement (in this case two filter sections of 1 m2 would have to be placed under each other). The used hydraulic load of filters was relatively low, compared to conventional wastewater trickling filters, but quite substantial, compared to sand or gravel filter beds. The increase in the hydraulic load would be possible and effective with respect to the removal efficiency of wastewater quality indicators in the conditions of uniform distribution of wastewater on the filter surface (inner cross section of the column).

3.2. Efficiency of Wastewater Quality Indicators Removal 3.2.1. Chemical Oxygen Demand The average value of inflow COD at preliminary stage was 717.8 ± 113.0 mg/L (n = 9). In experiment I: 284.1 ± 33.4 mg/L (n = 7), 290.0 ± 30.8 mg/L (n = 7), and 287.7 ± 31.2 mg/L (n = 7) for AH1, AH2, and SCR, respectively. In experiment II, average inflow values for COD were as follows: 537.2 ± 47.9 mg/L (n = 6) and 548.0 ± 46.3 mg/L (n = 6) for AH1 and AH2, respectively. The inflow COD values are presented in Tables1–3 and Figures3–5.

Table 1. Inlet and outlet COD and removal efﬁciencies at preliminary stage.

AH SCR Inlet Sample No. Outlet Efﬁciency Outlet Efﬁciency mg/L mg/L % mg/L % 1 430 132 69.3 264 38.6 2 617 63 89.8 72 88.3 3 691 120 82.6 200 71.1 4 680 72 89.4 164 75.9 5 740 197 73.4 151 79.6 6 681 301 55.8 186 72.7 7 1580 354 77.6 334 78.9 8 551 254 53.9 214 61.2 9 490 238 51.4 143 70.8 Avg. 717.8 ± 113.0 192.3 ± 34.1 71.5 ± 5.0 192.0 ± 25.1 70.8 ± 4.7

Table 2. Inlet and outlet COD and removal efﬁciencies at stage I.

AH1 AH2 SCR Sample Inlet Outlet Efficiency Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % mg/L mg/L % 1 204 43 78.9 204 30 85.3 204 40 80.4 2 155 E. n. d. 155 37 76.1 155 31 80.0 3 299 57 80.9 300 47 84.3 305 40 86.9 4 289 22 92.4 348 40 88.5 274 49 82.1 5 294 61 79.3 314 83 73.6 343 52 84.8 6 314 38 87.9 324 57 82.4 356 54 84.8 7 434 41 90.6 385 50 87.0 377 53 85.9 Avg. 284.1 ± 33.4 43.7 ± 5.7 85.0 ± 2.5 290.0 ± 30.8 49.1 ± 6.6 82.5 ± 2.1 287.7 ± 31.2 45.6 ± 3.3 83.6 ± 1.0 Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 16

COD were as follows: 537.2 ± 47.9 mg/L (n = 6) and 548.0 ± 46.3 mg/L (n = 6) for AH1 and AH2, respectively. The inflow COD values are presented in Tables 1–3 and Figures 3–5. The average efficiencies of COD removal at preliminary stage were as follows: 71.5 ± 5.0% (n = 9) for angel hair (AH) and 70.8 ± 4.7% (n = 9) for crushed material (SCR). The details related to COD removal efficiencies at preliminary stage are presented in Table 1 and Figure 3.

Table 1. Inlet and outlet COD and removal efficiencies at preliminary stage.

AH SCR Inlet Appl. Sci.Sample2021, 11, 5281No. Outlet Efficiency Outlet Efficiency8 of 17 mg/L mg/L % mg/L % 1 430 132 69.3 264 38.6 2 617Table 3. Inlet and outlet63 COD and removal89.8 efficiencies at stage II.72 88.3 3 691 120 AH182.6 200 AH2 71.1 Sample 4 680 Inlet72 Outlet89.4 Efficiency Inlet164 Outlet75.9 Efficiency No. 5 740 mg/L197 mg/L73.4 % mg/L151 mg/L79.6 % 6 681 1 685301 4555.8 93.4 695186 4672.7 93.4 2 685 54 92.1 690 62 91.0 7 1580 3 484354 10877.6 77.7 485334 6478.9 86.8 8 551 4 498254 10653.9 78.7 483214 7061.2 85.5 5 445 47 89.4 440 67 84.8 9 490 6 426238 5751.4 86.6 495143 8370.8 83.2 Avg. 717.8 ± 113.0Avg. 537.2 192.3± 47.9 ± 34.169.5 ± 12.0 71.5 86.3± 5.0± 2.7 548.0 192.0± 46.3 ± 25.165.3 ± 4.9 70.8 ± 87.5 4.7± 1.6

Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 16

Figure 3. Inlet and outlet COD and removal efﬁciencies at preliminary stage. Figure 3. Inlet and outlet COD and removal efficiencies at preliminary stage.

In experiment I, the average efficiencies of COD removal were as follows: 85.0 ± 2.5% (n = 6), 82.5 ± 2.1% (n = 7), and 83.6 ± 1.0% (n = 7), for AH1, AH2, and SCR, respectively. COD removal efficiencies at stage I are presented in Table 2 and Figure 4.

Table 2. Inlet and outlet COD and removal efficiencies at stage I.

AH1 AH2 SCR Sample Inlet Outlet Efficiency Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % mg/L mg/L % 1 204 43 78.9 204 30 85.3 204 40 80.4 2 155 E. n. d. 155 37 76.1 155 31 80.0 3 299 57 80.9 300 47 84.3 305 40 86.9 4 289 22 92.4 348 40 88.5 274 49 82.1 5 294 61 79.3 314 83 73.6 343 52 84.8

6 314 Figure 38 4. Inlet87.9 and outlet324 COD and removal57 efﬁciencies82.4 at stage356 I. 54 84.8 Figure7 4. Inlet434 and outlet 41 COD and90.6 remova l385 efficiencies 50 at stage87.0 I. 377 53 85.9 Avg. 284.1 ± 33.4 43.7 ± 5.7 85.0 ± 2.5 290.0 ± 30.8 49.1 ± 6.6 82.5 ± 2.1 287.7 ± 31.2 45.6 ± 3.3 83.6 ± 1.0 The average efficiencies of COD removal at stage II were as follows: 86.3 ± 2.7% (n = 6) for AH1 and 87.5 ± 1.6% (n = 6) for AH2. The details related to COD removal efficiencies at stage II are presented in Table 3 and Figure 5.

Table 3. Inlet and outlet COD and removal efficiencies at stage II.

AH1 AH2 Sample Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % 1 685 45 93.4 695 46 93.4 2 685 54 92.1 690 62 91.0 3 484 108 77.7 485 64 86.8 4 498 106 78.7 483 70 85.5 5 445 47 89.4 440 67 84.8 6 426 57 86.6 495 83 83.2 Avg. 537.2 ± 47.9 69.5 ± 12.0 86.3 ± 2.7 548.0 ± 46.3 65.3 ± 4.9 87.5 ± 1.6

Figure 5. Inlet and outlet COD and removal efficiencies at stage II.

The removal efficiencies of organic compounds expressed as COD were relatively high during all stages of the study (above 70% at the preliminary stage and above 80% at stages I and II). Similar results (especially compared to stages I and II), although for a different but indirectly related indicator—total organic carbon (TOC), obtained by Zhang Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 16

Figure 4. Inlet and outlet COD and removal efficiencies at stage I.

The average efficiencies of COD removal at stage II were as follows: 86.3 ± 2.7% (n = 6) for AH1 and 87.5 ± 1.6% (n = 6) for AH2. The details related to COD removal efficiencies at stage II are presented in Table 3 and Figure 5.

Table 3. Inlet and outlet COD and removal efficiencies at stage II.

Appl. Sci. 20216, 11, 5281 426 57 86.6 495 83 83.2 9 of 17 Avg. 537.2 ± 47.9 69.5 ± 12.0 86.3 ± 2.7 548.0 ± 46.3 65.3 ± 4.9 87.5 ± 1.6

Figure 5. Inlet and outlet COD and removal efficiencies at stage II. Figure 5. Inlet and outlet COD and removal efficiencies at stage II. The average efficiencies of COD removal at preliminary stage were as follows: The removal efficiencies71.5 ± 5.0% of(n =organic 9) for angel comp hairounds (AH) andexpressed 70.8 ± 4.7% as COD (n = 9) were for crushed relatively material (SCR). The details related to COD removal efficiencies at preliminary stage are presented high during all stagesin of Table the1 andstudy Figure (above3. 70% at the preliminary stage and above 80% at stages I and II). Similar Inresults experiment (especially I, the average compared efficiencies to of stages COD removal I and were II), asalthough follows: 85.0 for± a2.5% different but indirectly(n = relate 6), 82.5d± indicator—total2.1% (n = 7), and 83.6 organic± 1.0% carbon (n = 7), for (TOC), AH1, AH2,obtained and SCR, by respectively.Zhang COD removal efficiencies at stage I are presented in Table2 and Figure4. The average efficiencies of COD removal at stage II were as follows: 86.3 ± 2.7% (n = 6) for AH1 and 87.5 ± 1.6% (n = 6) for AH2. The details related to COD removal efficiencies at stage II are presented in Table3 and Figure5. The removal efficiencies of organic compounds expressed as COD were relatively high during all stages of the study (above 70% at the preliminary stage and above 80% at stages I and II). Similar results (especially compared to stages I and II), although for a different but indirectly related indicator—total organic carbon (TOC), obtained by Zhang et al. [7]— was about 89%. In contrast, Zhang et al. [11] obtained for this material much lower TOC removal efficiency, only about 70%. Higher efficiencies at stages I and II (comparing to the preliminary stage) resulted from a lower hydraulic load and a more even distribution of wastewater on the filter surface (periodic cleaning of sprinklers).

3.2.2. Biochemical Oxygen Demand According to the preliminary experiment result, it was found that septic tank effluent used in the experiment is relatively low in susceptibility to bio-decomposition, and therefore, the main experiment can be carried out. The COD/BOD5 ratio was 2.9 ± 0.2 and was comparable to ratios referred by Jó´zwiakowski [24], i.e., 2.6–3.1 for septic tank effluent collected at conditions of long retention time (7–11 days). The average value of BOD5 inflow at the preliminary stage was 311.1 ± 87.5 mg/L (n = 9). One record (measurement) was excluded from the analysis set as a gross error (enormous inlet value). During stages I and II, the BOD5 was determined occasionally with the aim to verify the COD/BOD5 ratio, indicated during the preliminary experiment. The BOD5 measurements during stages I and II were less accurate (compared to the preliminary experiment) due to the small sample volumes. Appl. Sci. 2021, 11, 5281 10 of 17

3.2.3. Total Phosphorus Removal

The average values of inﬂow Ptot at stage I were as follows: 23.5 ± 1.7 mg/L (n = 7), 22.6 ± 1.3 mg/L (n = 7), and 23.0 ± 1.4 mg/L (n = 7) for AH1, AH2, and SCR, respectively. At stage II, Ptot average inlet values were as follows: 6.9 ± 0.7 mg/L (n = 6) and 7.0 ± 0.7 mg/L (n = 6) for AH1 and AH2, respectively. The inﬂow Ptot concentrations are presented in Tables4 and5 and Figures6 and7.

Table 4. Inlet and outlet Ptot and removal efﬁciencies at stage I.

AH1 AH2 SCR Sample Inlet Outlet Efficiency Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % mg/L mg/L % 1 20.1 11.3 43.8 20.1 10.4 48.3 20.1 10.7 46.8 2 17.6 E. n. d. 17.6 11 37.5 17.6 10.8 38.6 3 21.3 9.3 56.3 21.8 8.8 59.6 22.4 9.6 57.1 4 26.5 10.5 60.4 25.3 10.6 58.1 26.2 11.5 56.1 5 29.2 12.7 56.5 24.2 12.8 47.1 26.2 13.2 49.6 6 28.5 13.1 54.0 27.6 12.2 55.8 27.5 12.6 54.2 7 21.4 10.2 52.3 21.5 9.2 57.2 20.8 10.1 51.4 Avg. 23.5 ± 1.7 11.2 ± 0.6 53.9 ± 2.3 22.6 ± 1.3 10.7 ± 0.6 51.9 ± 3.0 23.0 ± 1.4 11.2 ± 0.5 50.6 ± 2.4

Table 5. Inlet and outlet Ptot and removal efﬁciencies at stage II.

AH1 AH2 Sample Inlet Outlet Efﬁciency Inlet Outlet Efﬁciency No. mg/L mg/L % mg/L mg/L % 1 8.8 2.8 68.2 9 2.7 70.0 2 9.1 2.8 69.2 8.9 3.6 59.6 3 5.4 3.2 40.7 5.8 2.7 53.4 4 5.4 3.1 42.6 5.6 2.7 51.8 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 5.5 3.4 38.2 5.5 3.110 of 43.6 16 6 7.2 3.3 54.2 7 2.9 58.6 Avg. 6.9 ± 0.7 3.1 ± 0.1 52.2 ± 5.7 7.0 ± 0.7 3.0 ± 0.1 56.2 ± 3.6

Figure 6. Inlet and outlet Ptot and removal efﬁciencies at stage I. Figure 6. Inlet and outlet Ptot and removal efficiencies at stage I.

In experiment II, the average efficiencies of Ptot removal were as follows: 52.2 ± 5.7% (n = 6) and 56.2 ± 3.6% (n = 6), for AH1 and AH2, respectively. Ptot removal efficiencies at stage II were presented in Figure 7. Total phosphorus removal efficiencies obtained in this study were slightly higher than in studies made by Zhang et al. [11]—about 46%.

Table 5. Inlet and outlet Ptot and removal efficiencies at stage II.

AH1 AH2 Sample Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % 1 8.8 2.8 68.2 9 2.7 70.0 2 9.1 2.8 69.2 8.9 3.6 59.6 3 5.4 3.2 40.7 5.8 2.7 53.4 4 5.4 3.1 42.6 5.6 2.7 51.8 5 5.5 3.4 38.2 5.5 3.1 43.6 6 7.2 3.3 54.2 7 2.9 58.6 Avg. 6.9 ± 0.7 3.1 ± 0.1 52.2 ± 5.7 7.0 ± 0.7 3.0 ± 0.1 56.2 ± 3.6

Figure 7. Inlet and outlet Ptot and removal efficiencies at stage II.

Total phosphorus removal efficiencies of both types of material (angel hair and scraps) were very similar at both stages I and II. The obtained efficiencies are rather typical for single-stage trickling filters. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 16

Figure 6. Inlet and outlet Ptot and removal efficiencies at stage I.

Table 5. Inlet and outlet Ptot and removal efficiencies at stage II.

AH1 AH2 Sample Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % 1 8.8 2.8 68.2 9 2.7 70.0 2 9.1 2.8 69.2 8.9 3.6 59.6 3 5.4 3.2 40.7 5.8 2.7 53.4 4 5.4 3.1 42.6 5.6 2.7 51.8 5 5.5 3.4 38.2 5.5 3.1 43.6 6 7.2 3.3 54.2 7 2.9 58.6 Appl. Sci. 2021, 11, 5281 11 of 17 Avg. 6.9 ± 0.7 3.1 ± 0.1 52.2 ± 5.7 7.0 ± 0.7 3.0 ± 0.1 56.2 ± 3.6

Figure 7. Inlet and outlet Ptot and removal efﬁciencies at stage II.

Figure 7. Inlet and outlet Ptot and removal efficiencies at stage II. In experiment I, the average efficiencies of Ptot removal were as follows: 51.9 ± 3.0% (n = 6), 50.6 ± 2.4% (n = 7), and 53.9 ± 2.3% (n = 7), for AH1, AH2, and SCR, respectively. Total phosphorusPtot removal removal efficiencies efficiencies at stage Iof are both presented types in Table of 4material and Figure (angel6. hair and scraps) were very similarIn experiment at both stages II, the average I and efficienciesII. The obtained of Ptot removal efficiencies were as follows:are rather 52.2 ±typ-5.7% (n = 6) and 56.2 ± 3.6% (n = 6), for AH1 and AH2, respectively. Ptot removal efficiencies at ical for single-stagestage trickling II were filters. presented in Figure7. Total phosphorus removal efficiencies obtained in this study were slightly higher than in studies made by Zhang et al. [11]—about 46%. Total phosphorus removal efficiencies of both types of material (angel hair and scraps) were very similar at both stages I and II. The obtained efficiencies are rather typical for single-stage trickling filters.

3.2.4. Ammonium Nitrogen

The average values of inflow NNH4 at stage I were as follows: 100.1 ± 11.3 mg/L (n = 7), 93.5 ± 7.6 mg/L (n = 7), and 92.5 ± 8.9 mg/L (n = 7) for AH1, AH2, and SCR, respectively. In stage II, Ptot average inlet values were as follows: 2.8 ± 0.6 mg/L (n = 5) and 2.5 ± 0.6 mg/L (n = 5) for AH1 and AH2, respectively. One record (the last one) was rejected as a rough error (inflow concentrations: 0.13 mg/L for both columns and outflow concentrations: 1.37 mg/L and 1.35 mg/L for AH1 and AH2, respectively). The inflow NNH4 concentrations are presented in Tables6 and7 Figures8 and9.

Table 6. Inlet and outlet NNH4 and removal efﬁciencies at stage I.

AH1 AH2 SCR Sample Inlet Outlet Efficiency Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % mg/L mg/L % 1 64.4 0.99 98.5 64.4 0.33 99.5 64.4 0.53 99.2 2 75 E. n. d. 75 0.8 98.9 75 0.32 99.6 3 93 0.86 99.1 97 1.31 98.6 100 1.02 99.0 4 152 2.2 98.6 108 3.8 96.5 98 7.3 92.6 5 116 2.2 98.1 117 15.7 86.6 124 7.8 93.7 6 116 2.1 98.2 111 2.5 97.7 117 2.3 98.0 7 84 1.4 98.3 82 1.5 98.2 69 1.5 97.8 Avg. 100.1 ± 11.4 1.6 ± 0.3 98.5 ± 0.1 93.5 ± 7.6 3.7 ± 2.0 96.6 ± 1.7 92.5 ± 8.9 3.0 ± 1.2 97.1 ± 1.1 Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 16

3.2.4. Ammonium Nitrogen

The average values of inflow NNH4 at stage I were as follows: 100.1 ± 11.3 mg/L (n = 7), 93.5 ± 7.6 mg/L (n = 7), and 92.5 ± 8.9 mg/L (n = 7) for AH1, AH2, and SCR, respectively. In stage II, Ptot average inlet values were as follows: 2.8 ± 0.6 mg/L (n = 5) and 2.5 ± 0.6 mg/L (n = 5) for AH1 and AH2, respectively. One record (the last one) was rejected as a rough error (inflow concentrations: 0.13 mg/L for both columns and outflow concentrations: 1.37 mg/L and 1.35 mg/L for AH1 and AH2, respectively). The inflow NNH4 concentrations are presented in Tables 6 and 7 Figures 8 and 9. In experiment I, the average efficiencies of NNH4 removal were as follows: 98.5 ± 0.13% (n = 6), 96.6 ± 1.7% (n = 7), and 97.1 ± 1.1% (n = 7), for AH1, AH2, and SCR, respectively. NNH4 removal efficiencies at stage I are presented in Table 6 and Figure 8.

Appl. Sci. 2021, 11, 5281 12 of 17 Table 6. Inlet and outlet NNH4 and removal efficiencies at stage I.

AH1 AH2 SCR Sample Inlet OutletTable 7.EfficiencyInlet and outlet Inlet NNH4 and removalOutlet efficienciesEfficiency at stageInlet II. Outlet Efficiency No. mg/L mg/L % mg/L AH1mg/L % mg/L mg/L AH2 % 1 64.4 0.99 Sample98.5 64.4 0.33 99.5 64.4 0.53 99.2 Inlet Outlet Efficiency Inlet Outlet Efficiency 2 75 E. No. n. d. 75 0.8 98.9 75 0.32 99.6 3 93 0.86 99.1 mg/L97 mg/L1.31 %98.6 mg/L100 1.02 mg/L 99.0 % 4 152 2.2 198.6 3.7108 1.73.8 54.196.5 3.2 98 7.3 2 92.6 37.5 2 4.5 3 33.3 4.5 2.3 48.9 5 116 2.2 98.1 117 15.7 86.6 124 7.8 93.7 3 1.65 0.6 63.6 1.55 1.27 18.1 6 116 2.1 498.2 1.59111 1.112.5 30.297.7 1.6117 2.3 1.16 98.0 27.5 7 84 1.4 598.3 2.5382 0.951.5 62.598.2 1.7169 1.5 0.88 97.8 48.5 Avg. 100.1 ± 11.4 1.6 ± 0.3Avg. 98.5 ± 0.1 2.8 ± 93.50.6 ± 7.6 1.5 ± 3.70.4 ± 2.0 48.7 96.6± ±7.1 1.7 2.592.5± ±0.6 8.9 3.0 1.5 ±± 1.20.3 97.1 36.1 ± 1.1± 6.0

Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 16

Table 7. Inlet and outlet NNH4 and removal efficiencies at stage II.

AH1 AH2 Sample No. Inlet Outlet Efficiency Inlet Outlet Efficiency mg/L mg/L % mg/L mg/L % 1 3.7 1.7 54.1 3.2 2 37.5 2 4.5 3 33.3 4.5 2.3 48.9 3 1.65 0.6 63.6 1.55 1.27 18.1 4 1.59 1.11 30.2 1.6 1.16 27.5 5 2.53 0.95 62.5 1.71 0.88 48.5 Avg. 2.8 ± 0.6 1.5 ± 0.4 48.7 ± 7.1 2.5 ± 0.6 1.5 ± 0.3 36.1 ± 6.0 Figure 8. Inlet and outlet NNH4 and removal efﬁciencies at stage I. Figure 8. Inlet and outlet NNH4 and removal efficiencies at stage I.

In experiment II, the average efficiencies of NNH4 removal were as follows: 48.7 ± 7.1% (n = 5) and 36.1 ± 6.0% (n = 5), for AH1 and AH2, respectively. NNH4 removal efficiencies at stage II are presented in Table 7 and Figure 9.

Figure 9. Inlet and outlet NNH4 and removal efﬁciencies at stage II.

Figure 9. Inlet and outlet NNH4 and removal efficiencies at stage II.

Ammonium nitrogen removal efficiency was very high during experiment I and almost equal (taking into account average values) for both types of filling. Very high efficiency of ammonium nitrogen removal resulted from good oxygenation (due to natural ventilation) of the filter bed (a low hydraulic load of wastewater, relatively free air supply to both the surface zone and the bottom of filters). The results obtained in experiment I were comparable to results obtained by Zhang et al. [11] for sorbents (including xylit) with a pH between 7 and 9—about 87%. During experiment II, ammonium nitrogen removal efficiency was much lower, as compared to experiment I. It could result from very low inflow concentrations (greywater) and some minimum demand for nitrogen incorporated into bacterial cell biomass.

3.2.5. Total Suspended Solids The average values of inflow TSS at preliminary stage and stage I were as follows: 387.6 ± 73.5 mg/L (n = 9) and 107.3 ± 19.7 mg/L (n = 4), respectively (Tables 8 and 9). The average efficiencies of TSS removal at the preliminary stage were 42.1 ± 6.2% (n = 9) for angel hair (AH), and 70.2 ± 8.1% (n = 8, excluding the measurement of 2 July 2019, as a rough measurement error) for crushed material (SCR). The details related to TSS removal efficiencies at this stage are presented in Figure 10.

Appl. Sci. 2021, 11, 5281 13 of 17

In experiment I, the average efficiencies of NNH4 removal were as follows: 98.5 ± 0.13% (n = 6), 96.6 ± 1.7% (n = 7), and 97.1 ± 1.1% (n = 7), for AH1, AH2, and SCR, respectively. NNH4 removal efficiencies at stage I are presented in Table6 and Figure8. In experiment II, the average efficiencies of NNH4 removal were as follows: 48.7 ± 7.1% (n = 5) and 36.1 ± 6.0% (n = 5), for AH1 and AH2, respectively. NNH4 removal efficiencies at stage II are presented in Table7 and Figure9. Ammonium nitrogen removal efficiency was very high during experiment I and almost equal (taking into account average values) for both types of filling. Very high efficiency of ammonium nitrogen removal resulted from good oxygenation (due to natural ventilation) of the filter bed (a low hydraulic load of wastewater, relatively free air supply to both the surface zone and the bottom of filters). The results obtained in experiment I were comparable to results obtained by Zhang et al. [11] for sorbents (including xylit) with a pH between 7 and 9—about 87%. During experiment II, ammonium nitrogen removal efficiency was much lower, as compared to experiment I. It could result from very low inflow concentrations (greywater) and some minimum demand for nitrogen incorporated into bacterial cell biomass.

3.2.5. Total Suspended Solids The average values of inﬂow TSS at preliminary stage and stage I were as follows: 387.6 ± 73.5 mg/L (n = 9) and 107.3 ± 19.7 mg/L (n = 4), respectively (Tables8 and9).

Table 8. Inlet and outlet TSS and removal efﬁciencies at preliminary stage.

AH SCR Inlet Sample No. Outlet Efﬁciency Outlet Efﬁciency mg/L mg/L % mg/L % 1 654.2 129.60 80.2 654.2 24.20 2 218 142.00 34.9 218 17.60 3 348.9 283.10 18.9 348.9 202.00 4 343.9 247.40 28.1 343.9 13.50 5 351.9 210.30 40.2 351.9 109.50 6 642.8 318.00 50.5 642.8 375.00 7 665.1 313.00 52.9 665.1 208.80 8 57.7 43.40 24.8 57.7 184.80 9 205.5 105.60 48.6 205.5 90.60 Avg. 387.6 ± 73.5 199.2 ± 32.9 42.1 ± 6.2 387.6 ± 73.5 136.2 ± 39.7

Table 9. Inlet and outlet TSS and removal efﬁciencies at stage I.

AH1 AH2 SCR Sample Inlet Outlet Efficiency Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % mg/L mg/L % 1 66 7.30 88.9 66 5.2 92.1 66 21.70 67.1 2 82.2 E. n. d. 82.2 9.2 88.8 82.2 25.90 68.5 3 132.9 63.20 52.4 132.9 E. n. d. 132.9 22.40 83.1 4 148.1 127.30 14.0 148.1 9.4 93.7 148.1 13.80 90.7 Avg. 107.3 ± 19.7 65.9 ± 34.7 51.8 ± 21.6 107.3 ± 19.7 7.9 ± 1.4 91.5 ± 1.4 107.3 ± 19.7 21.0 ± 2.6 77.4 ± 5.7

The average efﬁciencies of TSS removal at the preliminary stage were 42.1 ± 6.2% (n = 9) for angel hair (AH), and 70.2 ± 8.1% (n = 8, excluding the measurement of 2 July 2019, as a rough measurement error) for crushed material (SCR). The details related to TSS removal efﬁciencies at this stage are presented in Figure 10. Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 16

Table 8. Inlet and outlet TSS and removal efficiencies at preliminary stage.

AH SCR Inlet Sample No. Outlet Efficiency Outlet Efficiency mg/L mg/L % mg/L % 1 654.2 129.60 80.2 654.2 24.20 2 218 142.00 34.9 218 17.60 3 348.9 283.10 18.9 348.9 202.00 4 343.9 247.40 28.1 343.9 13.50 5 351.9 210.30 40.2 351.9 109.50 6 642.8 318.00 50.5 642.8 375.00 7 665.1 313.00 52.9 665.1 208.80 8 57.7 43.40 24.8 57.7 184.80 9 205.5 105.60 48.6 205.5 90.60 Appl. Sci. 2021, 11, 5281 14 of 17 Avg. 387.6 ± 73.5 199.2 ± 32.9 42.1 ± 6.2 387.6 ± 73.5 136.2 ± 39.7

Figure 10. Inlet and outlet TSS and removal efficiencies at preliminary stage. Figure 10. Inlet and outlet TSS and removal efficiencies at preliminary stage. Appl. Sci. 2021, 11, x FOR PEER REVIEW The average efficiencies of TSS removal efficiency at stage I were highly14 variableof 16 The average efficiencies(14.0%–93.7%). of TSS The averageremoval TSS effi removalciency efficiencies at stage were I alsowere differentiated—on highly variable average for AH1, was 51.8 ± 21.6% (n = 3), for AH, 91.5 ± 1.4% (n = 3), and for SCR, 77.4 ± 5.7% (14.0%–93.7%). The average(n = 4). The TSS details removal related toefficiencies TSS removal were efficiencies also atdifferentiated—on stage I are presented in aver- Figure 11. age for AH1, was 51.8 ± 21.6% (n = 3), for AH, 91.5 ± 1.4% (n = 3), and for SCR, 77.4 ± 5.7% (n = 4). The details related to TSS removal efficiencies at stage I are presented in Figure 11.

Table 9. Inlet and outlet TSS and removal efficiencies at stage I.

AH1 AH2 SCR Sample Inlet Outlet Efficiency Inlet Outlet Efficiency Inlet Outlet Efficiency No. mg/L mg/L % mg/L mg/L % mg/L mg/L % 1 66 7.30 88.9 66 5.2 92.1 66 21.70 67.1 2 82.2 E. n. d. 82.2 9.2 88.8 82.2 25.90 68.5 3 132.9 63.20 52.4 132.9 E. n. d. 132.9 22.40 83.1 4 148.1 127.30 14.0 148.1 9.4 93.7 148.1 13.80 90.7 Avg. 107.3 ± 19.7 65.9 ± 34.7 51.8 ± 21.6 107.3 ± 19.7 7.9 ± 1.4 91.5 ± 1.4 107.3 ± 19.7 21.0 ± 2.6 77.4 ± 5.7

Figure 11. Inlet and outlet TSS and removal efficiencies at stage I. Figure 11. Inlet and outlet TSS and removal efficiencies at stage I. During the preliminary experiment, some anomalies related to inlet TSS concentrations were observed. Although no flow-rate-related problems were identified, these phenomena During the preliminarycould be theexperiment, result of dosing some tubes anomalies clogging byrelated suspended to inlet solids TSS or concentra- other unknown tions were observed.phenomena. Although During no flow-rat stages I ande-related II, the dosing problems tubes werewere controlled identified, (and shaken)these to phenomena could beprevent the result clogging. of dosing tubes clogging by suspended solids or other unknown phenomena. DuringThe average stages efficiencies I and of totalII, the suspended dosing solid tubes removal were were controlled not identified (and at stage II, and it was concluded from the preliminary stage and stage I that this indicator concentra- shaken) to prevent clogging.tion is highly changeable due to the potential excess biomass detachment. Some amounts The average efficienciesof suspended of solids total in thesuspende filter effluentd solid are rather removal unavoidable were due not to theidentified activity, growth, at stage II, and it was concludedand detachment from of somethe ofpreliminary the excessive stage biomass and inhabiting stage theI that filter. this The increaseindicator in con- concentration is highly changeable due to the potential excess biomass detachment. Some amounts of suspended solids in the filter effluent are rather unavoidable due to the activity, growth, and detachment of some of the excessive biomass inhabiting the filter. The increase in concentration and increased instability of total suspended solids in the effluent may also have been influenced by the development of Psychoda flies, which were observed periodically. The overall TSS removal efficiencies were significantly higher during stage I, as compared to the preliminary stage. The following factors can be indicated as the main reasons for the variable and quite low efficiency of TSS removal at the preliminary stage: non-uniform distribution of wastewater on the surface of the filters and a relatively high hydraulic load. As only a part of a whole wastewater facility system, i.e., trickling filter (filter columns), was investigated in the experiments, it is worth noting that under technical conditions (as per most types and kinds of conventional trickling filters), it is recommended to use secondary settlers to remove effluent in the form of excessive biomass periodically occurring in the filter (in the form of suspended solids).

3.2.6. Statistical Analysis No statistically principal differences (95% difference interval) between ‘angel hair’ and ‘scraps’ removal efficiencies of wastewater quality indicators were stated. The inability to confirm the statistical significance for the differences between the means of the paired sample was caused by the relatively high variability of individual values.

centration and increased instability of total suspended solids in the effluent may also have been influenced by the development of Psychoda flies, which were observed periodically. The overall TSS removal efficiencies were significantly higher during stage I, as compared to the preliminary stage. The following factors can be indicated as the main reasons for the variable and quite low efficiency of TSS removal at the preliminary stage: non-uniform distribution of wastewater on the surface of the filters and a relatively high hydraulic load. As only a part of a whole wastewater facility system, i.e., trickling filter (filter columns), was investigated in the experiments, it is worth noting that under technical conditions (as per most types and kinds of conventional trickling filters), it is recommended to use secondary settlers to remove effluent in the form of excessive biomass periodically occurring in the filter (in the form of suspended solids).

3.2.6. Statistical Analysis No statistically principal differences (95% difference interval) between ‘angel hair’ and ‘scraps’ removal efficiencies of wastewater quality indicators were stated. The inability to confirm the statistical significance for the differences between the means of the paired sample was caused by the relatively high variability of individual values.

4. Conclusions It should be noted that due to periodic leaching of biomass, it is recommended to use secondary settling tanks for biological deposits. The use of xylit for domestic wastewater (raw or pre-treated in a septic tank) treatment can be a promising solution taking into account both macro- and micropollutant removal. This material can be packed much more densely than was used in this study, which should increase its efficiency in the removal of wastewater quality indicators. Although the experiment (all stages) lasted 15 months and no xylit material clogging problems were observed, there is a need to verify the clogging process intensity (e.g., under technical conditions). Based on the results obtained, the following conclusions and recommendations can be proposed: • The average efficiencies of COD removal were over 70, 80, and 85% for preliminary stage, stage I, and stage II, respectively; • The effectiveness of TSS removal was strongly differentiated in the preliminary stage and stage I: 42% and 70% during the preliminary stage and 88, 91, and 65% during stage I; • The efficiency of ammonium nitrogen (NNH4) removal was very high at stage I: 97–99%, but relatively low at stage II: 36–49%; • Average total phosphorus removal efficiencies were practically equal at stages I and II: 51–54% and 52–56%, respectively; • The removal efficiency of wastewater quality indicators using both types of materials did not differ significantly; • The investigated hydraulic load of filters enables their use in filed scale a relatively compact treatment system (less than 1.0 m2 of surface area in plain view) for one homestead (four-person family); • The study demonstrated that xylit was a material highly effective in the removal of wastewater quality indicators, even during the initial period of its use (preliminary stage); • Using xylit as a trickling filter material it is recommended to take care about uniform distribution of wastewater on the filter surface. There is a need for further research on xylit material under technical conditions (field scale) to determine basic applicable conditions, e.g., hydraulic and organic load or packing density. Appl. Sci. 2021, 11, 5281 16 of 17

Author Contributions: Conceptualisation, M.S. and R.M.; methodology, M.S. and R.M.; validation M.S. and R.M.; formal analysis, M.S.; investigation, M.S. and R.M.; resources, M.S.; data curation, M.S. and T.N.; writing—original draft preparation, M.S. and T.N.; writing—review and editing, M.S. and R.M.; visualisation, M.S. and T.N; supervision, M.S.; project administration, M.S.; funding acquisition, M.S. and R.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received external funding: Project No. 005/RID/2018/19 under “Wielkopol- ska Regional Initiative of Excellence in the area of life sciences of the PoznańUniversity of Life Sciences”. The project was financed under the program of the Minister of Science and Education in 2019–2022. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request. Acknowledgments: The authors thank Krzysztof Ziewiecki and EKOTECH Company for the de- livery of research material (xylit) and the information related to it. The authors also thank Jolanta Zawadzka—a laboratory technician at the Department of Water and Sanitary Engineering—for her help in laboratory measurements and the Amica S. A. company, for renting a washing machine. Conflicts of Interest: The authors declare no conflict of interest.

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