Performance investigation of the slaughterhouse wastewater treatment facility: a case of Mwanza City Slaughterhouse, Tanzania

The present study engaged onsite operations and laboratory analysis for Mwanza City Slaughterhouse (MCS) wastewater to improve the ef ﬁ ciency of wastewater treatment of a newly installed facility. The MCS wastewater treatment facility is integrated with various units-biodigester, aeration unit, retention, clari ﬁ er, and a constructed wetland. During the initial runs, the MCS facility removed 87.5%, 92.2%, 43%, and 65.4% of ef ﬂ uent biochemical oxygen demand (BOD 5 ), chemical oxygen demand (COD), ammonium, and nitrate, respectively. After conducting effective plant operations for ﬁ ve months, the removal ef ﬁ ciencies of BOD 5 , COD, ammonium, and nitrate improved to 97.4%, 98.3%, 97.4%, and 97.6%, respectively. In the present study, the unit-by-unit performance values achieved as a result of alterations of the facility ’ s running conditions are presented. The MCS wastewater treatment facility was found to be energy-positive, as it produced an average of 158.2 m 3 biogas per day. This amount of biogas, if converted to electricity, would be suf ﬁ cient to run the facility operations. Generally, the MCS wastewater treatment facility attained the best performance as per design, achieving the ef ﬂ uent levels recommended by the Tanzania Bureau of Standards (TBS).


INTRODUCTION Background information
Meat industry generates huge volumes of wastewaters that come from cleaning of slaughterhouse facilities, meat processing, and cleaning of animal carcasses (Bustillo-Lecompte & Mehrab 2015). The volume of these wastewaters being released into the receiving environment has also increased over the years due to increased meat production to meet protein requirements of growing human populations (Emmanuel et al. 2016). Slaughterhouse wastewater contains biodegradable suspensions, colloidal particles, organic matter, fats, and cellulose that usually contribute to elevated levels of COD and BOD (Shujun et al. 2015). These materials can eventually reduce the amount of dissolved oxygen (DO) in the receiving aquatic environments (Sunder & Satyanarayan 2013). Thus, slaughterhouse wastewater requires considerable treatment to eliminate environmental contaminants before it is discharged into the receiving aquatic environments (Irshad et al. 2015).
The present study dealt with wastewater treatment processes at the newly installed MCS wastewater treatment facility in Tanzania. Before the MCS wastewater treatment facility was installed, the effluents used to be released untreated to the nearby receiving waters that empty into Lake Victoria. The Mwanza City Council (MCC) strived to find funds to resolve pollution problems related to industrial effluents that fed into Lake Victoria. The World Bank (WB) funded the construction of the slaughterhouse wastewater treatment facility through the Lake Victoria Environmental Management Project (LVEMP).
The LVEMP then contracted the Nelson Mandela African Institution of Science and Technology (NM-AIST) to design and supervise the construction of a slaughterhouse wastewater treatment facility for MCS. The design flow rate of wastewater into the facility was 130 m 3 per day. Other design parameters for influent quality of the MCS wastewater treatment facility, with their values indicated in parentheses, were as follows: pH (7.5), wastewater colour (10750 Pt-Co), total suspended solids (TSS) (9,700 mg/L), BOD 5 (1,200 mg/L), COD (4,500 mg/L), NH 3 -N (65 mg/L), SO 2À 4 (370 mg/L), and faecal coliform (2 Â 10 7 CFU/100 mL). The mean initial two-month performance after construction of the MCS wastewater treatment facility revealed that effluent contaminant levels were above the TBS regulation (Table 1).

Facility design considerations
The MCS wastewater treatment facility was designed with several units e.g. pretreatment unit (screening, oil and fat/grease trap and buffer tank), biodigester unit (stirred batch upflow anaerobic sludge blanket (UASB)), advanced treatment (aeration tank and clarifier) and a polishing step (constructed wetland). Also, the facility has subcomponents e.g. a biogas holder and a sludge drying bed. This makes it one of the novel systems that has, so far, not been extensively studied. During the study, the MCS wastewater treatment facility was energy positive because the daily energy consumption ranged between 50 and 65 kWh while the daily biogas production ranged from 220 to 250 kWh, if converted into electricity. However, at the time of the study, the biogas produced was not used for power generation because the utilities for power production were yet to be procured and installed. Furthermore, the initial production of biogas from the MCS wastewater treatment facility was below the estimated potential of 200 m 3 per day. Therefore, the aim of the present study was to investigate the performance of the MCS wastewater treatment facility by taking into account the factors that affect operational efficiencies.

Study area
The MCS wastewater treatment facility is located along Musoma Road, Mahina Ward in Mwanza, Tanzania ( Figure 1). The location lies below the equator between latitudes 2°and 4°south and longitude between 32°and 35°east of Greenwich. The city of Mwanza is located at the southern shores of Lake Victoria and has a population of approximately four million; according to the national census of 2012.

Treatment system design
The system was designed to have the following parameters: Feed flow rate, Q ¼ 65 m 3 /h; the dimensions of batch stirred biodigester were: diameter of 10 m, total height of 8.50 m with water level height of 6.62 m, a gas collector height of 1.38 m and a free body space of 0.50 m. The biodigester was designed to degrade 4,500 mg/L of influent COD, BOD 5 of 1,200 mg/L, TSS of 9,700 mg/L and NO À 3 of 605 mg/L. The designed hydraulic detention time, τ, was 4 days. The aeration tank has of diameter of 7 m, and height of 6.5 m with water level 6.0 m, aeration volume of 231 m 3 and designed hydraulic detention time, τ, of 1.8 days.
Wastewater treatment at the MCS The MCS wastewater treatment facility was operated as a semi-batch system, providing an intermittent flow of wastewater into the different units of the plant up to the constructed wetland ( Figure 2). The facility was designed to match its operation hours taking into account the fact that slaughter activities normally happen between 3 and 6 AM. The generated wastewater was quickly transferred into the biodigester to minimize biomethanation. Pumping into the biodigester was usually done for 1-2 h. The biodigester was fed from the bottom up. From the biodigester, the wastewater was transferred by gravity to be further treated in the aerated tank. Aeration was done for 12 h, then stopped to allow development of anoxic conditions for another 12 h. This mode of operation assisted in the denitrification of NO À 3 produced in the aerator during the nitrification of ammonium. The aeration tank was designed to receive 85 mg/L of NH þ 4 . Accordingly, the blower was designed to supply 48 kg/h of air for 12 h during the nitrification process.
After the aeration unit, there was a retention tank and a pump that continuously fed the rest of the units. During fieldwork for the present study, the MCS wastewater treatment facility was receiving an average amount of wastewater of 32.7 m 3 per day from carcass and meat washing as well as the slaughterhouse floor cleaning due to small number of animals slaughtered per day. Animals slaughtered at the MCS facility included goats, cattle, and sheep. In the course of the present study, the wastewater samples collected were found to contain high amounts of large solids from undigested offal. The solids were removed from the system using coarse and fine bar screens of 20 mm and 7.5 mm spacing, respectively, at the preliminary treatment stage. The wastewater from the aeration tank was then transferred to the retention tank before being continuously pumped into the clarifier. The clarifier's role was to separate the solids from the aeration tank from the wastewater before the water entered the polishing step. The sludge that accumulated in the clarifier was transferred into the sludge drying beds; later used as organic fertilizer for agricultural purposes. The dimensions of the Uncorrected Proof sludge drying bed were: Area of 42 m 2 , four (4) compartments each with the length of 3.75 m, width of 3.75 m (which made a total length of 16.5 m and width of 3.75 m), and height of 1.2 m. Inlet diameter channel was 100 mm terminating at 300 mm above the sand surface. Effluents from the clarifier were conveyed by gravity to the constructed wetland (CW), which was used as a polishing unit due to its ability to remove the remaining nutrients, organic matter, and suspended materials from wastewater. The constructed wetland was divided into two cells each with dimensions of 30 m length, 10 m width and 1 m depth. The wetland effective treatment depth was 0.5 m and granite gravel packing was of 20 mm size with porosity of 0.4. The daily influent to each of the constructed wetland cells was around 16 m 3 . During the course of the present study, the constructed wetland was observed to have a retention time of around 3.3 days.

Onsite measurements
Onsite measurements for EC, pH, TDS, temperature, and DO were carried out using a multiparameter probe (Palintest MACRO 900). Wastewater turbidity was measured using a turbidimeter (Palintest 09011150103). Wastewater and biogas volumes were recorded daily using a mass flow totalizer (GFT-110A). Biogas production was recorded daily whereas biogas composition was analyzed weekly. Biogas composition was determined using a gas analyzer (Geotech BIOGAS 5000). The facility's power consumption was determined using an electrical meter (EDMI EUPR-1232-1100) and a sub-meter (EM 0026-JC).

Wastewater sample collection
Daily amount of slaughterhouse wastewater produced at the MCS facility required treatment before discharging into the aquatic environment. In the present study, the analysis of influent and effluent wastewater was carried out. Wastewater sampling and analysis were done for five consecutive months. Performance evaluation period was started after two months of the trial runs where the plant performance observed was as shown in Table 1. In this period, a total of 112 samples were taken and analyzed.
Duplicate wastewater samples from influent and effluent of each treatment unit were collected in 500 mL plastic bottles. After collection, the samples for COD, NH þ 4 , NO À 3 and NH 3 analyses were acidified using sulfuric acid to a pH below 2 to deactivate microbial activities while another sample was not acidified as they could be transferred to reach the NM-AIST laboratory within 24 hours while packed in ice-packed cool boxes and kept at a temperature below 4°C.

Laboratory analysis
The determination of NH þ 4 , NO À 3 and NH 3 concentrations was done using a spectrophotometer (Hach DR-2800™). The analysis of NH 3 and NH þ 4 using Nessler method as detailed by Jeong et al. (2013). The analysis of NO À 3 was done using the Cadmium reduction method. The COD was determined using a HI-839800 Thermo-Reactor (HANNA Instruments). These environmental contamination indicators were analyzed as per standard methods for examinations of water and wastewater (APHA 2012). Titration method was used to measure the soluble volatile fatty acids (VFAs) and alkalinity in the biodigester. The analysis of a five-day BOD was determined through incubation (OxiTop ® IS12). The TSS was determined at a temperature of 105°C in a drying oven (BINDER GmbH FD 56 E3). The total volatile solids (TVS) and volatile suspended solids (VSS) were quantified following standard methods at a temperature of 550°C inside a muffle furnace (Cole-Parmer Stable Temp 1,100°C Box Furnace: CBF Series). The weight of dry solid samples was determined using a weighing balance (CY 204 S/N 15201586). Filtration of the slaughterhouse wastewater was done using a filtration pump (WELCH 2546C-02B) combined with a conical flask (Pyrex 580913 PORO 3). The faecal coliform count was determined in triplicates where the Petridishes of MacConkey agar containing 0.1 mL sample on filters (11406 ø 47 AC 1502023 0.45) were inoculated and incubated at 44.5°C for 24 h before counting.

General operational conditions
The present study was carried out for a period of five months, in the months of February to June 2019. The daily amount of wastewater fed into the biodigester at the MCS averaged to 32.7 m 3 . This wastewater volume, produced through animal slaughter and related activities, was but a fraction of the design value of 130 m 3 per day. The difference was attributable to the lower number of animals that was being slaughtered per day compared to the design capacity. The design capacity was for the facility to slaughter about 750 animals per day. During the course of the present study, only about 250 animals were being slaughtered per day. The effluent from the biodigester and clarifier was fed into the retention tank with a holding capacity of 130 m 3 per day installed with a pump that continuously fed the clarifier and the constructed wetland at a rate of 5.42 m 3 /h.
Wastewater temperature, pH and DO were measured onsite ( Table 2). The present study observed that the MCS wastewater treatment facility was operating at a temperature of around 26.3 + 0.3°C. This ambient temperature was lower than the one recommended in other studies i.e. a mesophilic temperature ranging from 30 to 40°C for essential enhancement of treatment efficiency as well as biogas production (Tsegaye et al. 2018). Thus, treatment efficiency of the MCS bioreactor can be significantly improved by raising this temperature either using solar heating (Kakaç & Pramuanjaroenkij 2016) or using part of the biogas generated to heat the incoming wastewater before entering the biodigester. At the time of the present study, these improvements were not possible. However, this has remained to be a recommendation for further improvement.
The present study also observed that the MCS wastewater treatment facility operated at a pH of approximately 7.2 + 0.1 that was within the optimal range for the bioreactors and is usually controlled by the VFA-to-alkalinity ratio. A pH range of 6.5-8.0 is known not to inhibit methanogenic bacteria during biogas production (Reis et al. 2016).
The DO concentrations ranged from 0.29 to 3.82 mg/L ( Table 2). The DO values of 0.29, 0.24, 3.82 and 1.79 mg/L were recorded in the buffer tank, biodigester outlet, aeration tank and constructed wetland, respectively. In comparison, the anticipated design DO values were 0.21, 5.0 and 2.0 mg/L for the buffer tank, aeration tank and constructed wetland, respectively. The low DO in the aeration tank compared with the anticipated 5 mg/L was probably due to lower air supply from the blower of around 41 kg/h than the anticipated supply of air of 48 kg/h. DO in anaerobic digesters may be caused by factors such as high mixing rates, high recirculation rate, and too much loss of activated sludge (Kato et al. 1997;Botheju & Bakke 2011). Conklin et al. (2007) studied the influence of DO on anaerobic digestion processes and found that a supply Uncorrected Proof of 3-4 mg/L of DO led to a 27% of active methanogenesis. These researchers concluded that a shortterm oxygen exposure did not significantly reduce methanogen activity. However, a continuous oxygen exposure was found to negatively affect the methanogenic biomass activity. In spite of the negative influence of oxygen exposure, researchers found no effect on long-term digester performance in terms of the biogas production rate. Therefore, for the MCS facility, it is important to monitor DO loadings into the digester to improve the long-term methanogenic activity of the facility.

Effects of agitation on biogas production
The effects of agitation time on the biodigester was investigated for zero to 6 h of agitation. The effects caused by both biodigester agitation duration and influent wastewater volume on the amount of biogas produced at the MCS wastewater treatment facility have been indicated (Table 3). Each of the agitation time was run once per day for 7 days and biogas produced during that period was recorded daily. With no agitation in the system and when the average volume of feed was 23 m 3 / day, the biogas produced was 145 m 3 per day. With one hour of constant agitation and an average influent feed of 20 m 3 per day, about 230 m 3 of biogas was produced. Increasing the hours of agitation to two at an average influent feed of about 19 m 3 per day, continued to lower the volume of biogas produced. Similarly, when agitation time was increased to four hours at an influent volume of about 22 m 3 per day, a dramatic reduction in biogas production was observed. A further increase in the number of agitation hours to six at an influent volume of about 22 m 3 , resulted in a further decrease in biogas production. Thus, the best biogas production occurred when the agitation time of 1 h was applied. It should be noted that before the start of the present study, the biodigester used to be agitated for 4 h per day. Agitation time of 1 h at a rate of 30 rpm was therefore recommended for improved biogas production. Agitation duration and speed have been linked to biogas production in a previous study (Aworanti et al. 2017). It has also been found by other researchers that a gentle biodigester agitation distributes uniformly the substrates to form a uniform suspension of solid and liquid parts, prevents foam formation and improves biogas production through fermentation processes (Lemmer et al. 2013).

Biodigester unit performance
The biodigester performance for the removal of important environmental pollutants has been indicated (Table 4). Transformation of N and N-compounds and related mass balance explanations are found in Equations (1)-(7) below. Alkalinity within an acceptable level is known to favour biogas production through maintenance of pH (Prabhudessai & Mutnuri 2013;Jung et al. 2019). An acidic environment in the biodigester is inhibitive in terms of biogas production (Sakar et al. 2009;Lee et al. 2019). Thus, for optimal biogas production, maintenance of alkalinity as CaCO 3 within a favourable range is important. NH þ 4 and alkalinity can be expected to increase in a well-performing wastewater treatment system as a result Uncorrected Proof of protein breakdown to NH 3 , which further combines with CO 2 to form NH 4 (HCO 3 ) (Equation 1) (Sunirat 2016). Likewise, for well-performing wastewater treatment systems, the VFAs should be expected to decrease in the biodigester because they become consumed by the methanogens in the methanogenic phase. However, in the present study, the alkalinity level was decreasing while the VFAs was increasing with time indicating poor performance in the treatment system which might be attributed to many factors such as retention time and agitation frequency, to mention a few. The VFAs-to-alkalinity ratio during the process increased from 0.13 to 0.3 (Table 4) which indicates that the buffering system was overloaded by the increase of VFAs. However, the VFA-to-alkalinity ratio under the present study was still in the acceptable range as per Shujun et al. (2015) who indicated that for a well-working digester, the VFA-to-alkalinity ratio falls between 0.3 and 0.4.
In the present study, at pH values of 7.2 the NH 3 to NH þ 4 ratio was about 0.5 (Table 4). This was consistent with a recent study that investigated ammonia levels in liquid phase during anaerobic digestion (Mutegoa et al. 2020). In wastewaters, ammonia exists, primarily, in two forms: the charged ammonium ion and the uncharged aqueous ammonia. This coexistence is highly pH-and temperature-dependent. The uncharged ammonia component is known to be more toxic than its charged counterpart because of its lipophilicity and ability to traverse biological membranes. At a pH range between 7 and 12 both the charged and uncharged species of ammonia are known to exist in wastewater at varying percentages (Caicedo et al. 2000;Körner et al. 2001;Philippe et al. 2011). Dissolved uncharged ammonia increases with increasing pH and temperature. At pH below 7, virtually, all ammonia is expected to exist as soluble ammonia gas. In the present study, at a pH of 7.2, the measured ammonia concentration was higher than expected and could be considered inhibitive. The cause for this high ammonia concentration is unknown. However, a study by Jeong et al. (2013) pointed out the deficiency of titrimetric methods in estimating the concentration of ammonia species in wastewater, especially when 'hindering' ions such as Mg, Cl, and Fe are present in high concentrations. It is possible that ammonia was overestimated in the present study due to the fact that, during the study, there were no apparent toxicity indications in the system as evidenced by the amount of biogas produced. To this point, a recommendation is thus made for a further study to examine the causes of the reported high concentration of ammonia.
A relatively high i.e. .60% removal efficiency was achieved in the biodigester unit for BOD 5 , COD, TSS, nitrate, and turbidity (Table 4). The high COD removal efficiency could be due to the biodigester's capacity to remove chemical contaminants through treatment processes and settleable sludge. As de Mes et al. (2003) reported, for cow slurry, the soluble COD of 25% inside the biodigester Uncorrected Proof could be converted into biogas due to increased circulation of water forming a well-settleable sludge.
In the present study, the composition of biogas resulting from COD transformation was as follows: methane (70.3%), carbon dioxide (29.2%) and other gases (0.5%). In the biodigester, there was a net production of NH þ 4 . This situation may be attributed to anoxic conditions in the biodigester, which led to the net formation of NH þ 4 through dissimilatory reduction of NO À 3 to form NH þ 4 and the anoxic fermentation of organic N to form NH þ 4 (Behrendt 2014) (Equations 2-3).
Under the anoxic conditions in the biodigester, NO À 3 is also removed through denitrification process as suggested by Sheng et al. (2013). In nature, denitrification can take place in both terrestrial and aquatic ecosystems. Denitrification is usually facilitated by a broad variety of denitrifying bacteria which degrade the organic material by using NO À 3 in the absence of oxygen as indicated in Equation 4 (Roy & Conrad 1999). The loss of NO À 3 can also be a result of a process in which NO À 3 is reduced to nitrogen gas (Equation 5). Typically, denitrification occurs in anoxic environments, where the concentration of dissolved and freely available oxygen is depleted. In these areas, NO À 3 or NO À 2 can be used as a substitute terminal electron acceptor instead of oxygen, a more energetically favorable electron acceptor (Equation 5). The terminal electron acceptor is a compound that gets reduced in the reaction by receiving electrons. The complete process can be expressed as a net balanced redox reaction, where NO À 3 gets fully reduced to N 2 as discussed by Sheng et al. (2013).
In Table 4, it is indicated that NO À 3 was still high in the biodigester. In the present study, it was observed that biodigester agitation was done intermittently. Due to this intermittent agitation, there was an improper separation of solids. Improper separation of solids may have led to increased formation of NO À 3 in the biodigester. Sources of NH þ 4 vary including the hydrolysis of urea (Equation 6) and undigested protein degradation; the latter source is slow and of secondary importance. NH þ 4 is further transformed to nitrite and nitrate by autotrophic microorganisms as indicated in Equation (7). During the transformation of NH þ 4 into nitrite, a greenhouse gas i.e. N 2 O is usually formed as an intermediate (Sommer et al. 2006). The formation of nitrous oxide has thus raised great interest in the study of nitrification.
Treatment processes in the biodigester were energy-positive, involving simple mechanisms shown in Equations (4)-(7). The expected design and actual performance of the biodigester were satisfactory (Table 4) because, for all parameters, the actual efficiency was lower than the design by an error margin of ,15%. Despite these discrepancies in performance, UASB systems, such as the one that was investigated in the present study, are increasingly becoming a promising technology for treating slaughterhouse wastewaters with reported efficiencies !84, !77% and !81% for BOD 5 , COD and TSS, respectively (Mittal 2006). This implies that the efficiency of the studied biodigester can be further improved.

Aeration tank performance
The aeration tank (AT) performance in terms of percentage removal for COD (52.4%), BOD 5 (51.6%), TSS (63.6%), NH þ 4 (48.2%), faecal coliform (46.6%), NO À 3 (58.3%), NH 3 (66.5%) and turbidity (66.9%) has been given ( Table 5). The aeration system was run for 12 h a day. Compared to the performance of the biodigester (Table 4), the aeration tank consumed the influent NH þ 4 ( Table 5). The aeration tank influent NH þ 4 was oxidized to NO À 2 and NO À 3 in the system using Nitrosomonas and Nitrobacter bacteria, respectively. Furthermore, the denitrification processes during no-aeration hours may have caused significant removal of nitrate. In the aeration tank, treatment processes took place in a linear manner (Equations 8-11). Equations (8) and (9) show processes that occurred during the 12 h of aeration. In Equations (10) and (11), ammonia was acidified using microbes through denitrification processes in the other 12 h of the day with no aeration.
HNO 2 þ 1=2O 2 À ! Nitrobacter HNO 3 þ Biomass (11) In comparison to design, removal efficiencies in the aeration step were satisfactory (Table 5). With the exception of COD, removal efficiencies for each contaminant were either better than the anticipated design values or within a 10% error. The COD removal had a 20% error.

Constructed wetland performance
When compared to the initial two-month performance (Table 1), the CW achieved better removal efficiencies after the five months of study (Table 6). At the beginning of the present study, it was observed that plants (Cyperus papyrus sp.) were not well established in the CW. This may have led to low performance in contaminant removal from wastewater. Also, it was observed that after a well-established growth of such plants in the CW, there was a remarkable improvement in the removal of faecal coliform, organics, and nutrients from slaughterhouse wastewater (Table 6). Microbes and plant roots are capable of removing organic compounds under both aerobic and anaerobic conditions. The microbiology of the slaughterhouse wastewater is delicate and complex, involving several bacterial groups, each with own optimum working conditions. It is possible that microbes in the CW were sensitive to some process parameters including alkalinity, pH, VFAs, temperature, etc. Thus, in the present study, these process parameters either controlled plant-related ecological functions or inhibited specific bacterial groups. Various plant and microbial processes are known to stabilize soils, vegetation, and other assemblages in the CW and could indeed support the reduction of nutrients and pathogens in the slaughterhouse wastewater (Vymazal 2010). Operational processes in the constructed wetland are expressed in Equation (12).
Organic Carbon þ O 2 À ! Heterotrophs CO 2 þ Biomass (12) The two compartments of the CW were designed to treat 65 m 3 of wastewater per day. In the present study, the CW was observed to treat 42 m 3 of wastewater volume per day. The expected effluent quality levels were: COD (60 mg/L), BOD 5 (30 mg/L), TSS (100 mg/L) and NO À 3 (20 mg/L). The actual performance of the constructed wetland was excellent because the maximum error of the actual efficiencies was close to 2% compared to the design efficiencies (Table 6). Other studies on the slaughterhouse wastewater using CWs provided similar results (Vymazal 2010;Paschal et al. 2017). A study conducted in Uganda revealed that a CW could efficiently remove the following contaminants: COD (71%), BOD (71%) and NO À 3 (76%) (Odong et al. 2013). In the present study, the CW removed the measured contaminants with efficiencies .78% (Table 6).

Performance of the integrated system
The overall performance of the biodigester-constructed wetland system has been provided ( Table 7). The present study shows that the integrated biodigester-CW system performed well in the removal of COD (98.3%), BOD 5 (97.4%), TSS (99.6%), NH þ 4 (191.0%), faecal coliform (99.1%)NO À 3 (93.3%), NH 3 (99.1%) and turbidity (99.9%). Levels of NH 3 and NO À 3 were reduced at the aeration stage by the nitrification and denitrification processes, respectively. It could be that the good performance of the biodigester-CW system was due to the presence of intermediate units which were performing complementary treatment tasks. Previous research found that for soluble COD removal, good bioreactor operations were attributed to variations of solids settling in slaughterhouse wastewater (Manjunath et al. 2000). The MCS treatment system was designed to remove 99, 98, and 73% of COD, BOD 5 and NO À 3 , respectively. The combined biodigester-CW system of the MCS wastewater treatment facility was able to remove all contamination indicators with an efficiency of .97% and produced effluents quality that fell within the TBS limits (Table 7).
Furthermore, the MCS wastewater treatment facility was designed to produce sludge volume of about 19,400 and about 5,700 m 3 per year from the biodigester and aeration tanks, respectively. The present study, the MCS facility was found to produce sludge volume of 15,800 m 3 and 5,300 m 3 per year from the biodigester and aeration tanks, respectively. Sludge produced at the MCS facility is usually applied as fertilizer to boost plant production in nearby agricultural fields.

Biogas production
In the present study, the average biogas production at the MCS facility was 158.2 m 3 per day. This high biogas production was probably caused by the presence of organic materials required for anaerobic bacteria as substrates for methanogenesis processes. The substrates present in the slaughterhouse wastewater are known to have adequate nutritional requirements for anaerobic bacteria to form new cells and act as energy sources (Anahita et al. 2019). Degradation of organic materials in the MCS biodigester to produce biogas can be attributed to the consortia of anaerobic bacteria under favourable conditions (Shah et al. 2017). Also, factors such as favourable pH, temperature, and VFA-toalkalinity ratio are known to stimulate the anaerobic bacteria to digest the liquid and cellulosic material in the slaughterhouse wastewater during the fermentation process (Jain et al. 2015).

Energy consumption
The electrical energy consumption for MCS wastewater treatment facility was observed to range between 50 and 65 kWh per day. Energy-consuming activities included the feeding of slaughterhouse wastewater into the digester, agitation of the biodigester, running of the aeration system, clarifier feeding and facility lighting. The present study found that these activities consumed electricity up to 1,950 kWh per month. This amount of energy was sometimes too costly for the MCC to afford and failed the continuity of operations at the treatment facility. However, the amount of biogas produced per day by the MCS wastewater treatment facility, if converted to electricity, would be enough to power the facility (Figure 3). Uddin et al. (2016) reported that 2.5 kWh electrical energy can be generated from one cubic meter of biogas. Therefore, the daily biogas produced at MCS can satisfy the plant's power requirements if converted to electricity using a biogas-run generator.

Biogas composition
The average biogas composition was as follows: CH 4 (70.3%), CO 2 (29.2%), O 2 (0.5%) and other gases of NH 3 (130 ppm) and H 2 S (120 ppm) per day (Table 8). Normally, biogas composition is dependent on the feedstock type and the activity of the consortia of anaerobic bacteria involved in the digestion process. The usual biogas composition from anaerobic digestion of organic-rich substrate includes CH 4 (50-75%), CO 2 (25-45%), O 2 (0-2%), NH 3 (0-1%) and H 2 S (0-1%) (Shah et al. 2017). Generally, the MCS wastewater treatment facility produced a high amount of biogas. However, the facility has more biogas production potential than it was producing during the course of the present study.