Performance evaluation of integrated Upflow Anaerobic Sludge Blanket reactor with trickling filter used for municipal wastewater treatment and effluent reuse potential for agriculture

Abstract

Effluent reuse is a rapidly growing field of research where assessing the quality of effluent is one of the focus areas. This research examines the viability of using wastewater in agriculture by testing an integrated Upflow Anaerobic Sludge Blanket (UASB) reactor with a trickling filter (TF) system during the dry season. Compliance monitoring was conducted for 30 days from May 11 to June 9 of 2021. Samples were collected, handled, and analyzed following standard wastewater analysis procedures for biological oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), volatile suspended solids (VSS), cations, anions, heavy metals, E. coli, and helminth egg. The UASB-TF system in Kality wastewater treatment performed well in removing COD, BOD₅, and TSS with average removal rates of 80.5%, 82.9%, and 80.9%, respectively, compared well with similar treatment configurations. The effluent quality satisfied the national inland discharge limit with a residual concentration of 125.1 mg/L for COD, 61.7 mg/L for BOD₅ and 85.8 mg/L for TSS. On the other hand, high concentrations of chromium, nitrate-nitrogen, and helminth egg count restricted effluent reuse for agricultural purposes due to high health risks and environmental contamination. We found out that discharging industrial sewage into the domestic sewer network could inhibit microbial growth and affect the biological treatment processes. Furthermore, adopting integrated treatment systems in developing countries might face operational challenges and monitoring nitrate, helminth egg, and heavy metals would help provide timely operational feedback. An appropriate tertiary treatment unit—constructed wetlands or polishing ponds—is therefore needed to be introduced to ensure effluent reuse for agricultural purposes.

Introduction

Agriculture is the biggest user of water and is facing scarcity due to the rising population and the effect of climate change. According to FAO (2019), agriculture consumes 70% of freshwater withdrawals, with low-income nations accounting for 90% of the total. Population expansion and climate change have increased food consumption, limiting freshwater supply and prompting the search of alternative water sources for agriculture in arid and semi-arid regions (Pastor et al. 2019). Water reuse—also known as water recycling or water reclamation—is becoming a strategic alternative and involves reclaiming water from different sources, treating it, and reusing it for beneficial purposes such as agriculture, irrigation, potable water supplies, groundwater replenishment, industrial processes, and environmental restoration. Wastewater reuse is becoming a strategic alternative for irrigation water supply, however only around 25% of the present wastewater production is treated (Jones et al. 2021). This study estimates global wastewater production at 359.4 × 109 m³/yr, of which 63% (225.6 × 109 m³/yr) is collected and 52% (188.1 × 109 m³/yr) is treated.

The United Nations (UN) has been committed to implementing wastewater reuse worldwide to meet the Sustainable Development Goals (SDGs) by 2030 (United Nations 2016). Advances in wastewater treatment and the increasing number of wastewater treatment plants producing large amounts of treated effluent have increased the potential for water reuse, particularly in urban areas. The global reuse of treated wastewater (TWW) for agriculture varies from 1.5 to 20% (Sato et al. 2013; Ungureanu et al. 2018; Jones et al. 2021). The reuse of TWW is a viable strategy in regions with water scarcity, such as China, Israel, Tunisia, and Chile (Yi et al. 2014; Bedbabis et al. 2014; Reznik et al. 2017; Vera-Puerto et al. 2022). The use of wastewater for irrigation provides essential macro- and micronutrients in the form of nitrogen (N), phosphorus (P), potassium (K), iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn), thus significantly improving soil nutrient content (Morgan and Connolly 2013; Hao et al. 2022; Zidan et al. 2024). The morphological development of plants is a clear indication of the value of the nutrients that treated sewage provides. Long-term experience from Israel demonstrated that over 87% of TWW generated is reused for agricultural irrigation (Marin et al. 2017), promoting extensive use of this untapped resource in water-scarce regions. However, the quality of the effluent remains a challenge for most wastewater treatment plants in low-income countries, hindering the reuse of TWW (Lazarova 2013; Gashaye 2020).

There is growing concern about the health and environmental risks associated with using treated wastewater (TWW) for irrigation. Exposure to pathogens in humans, effects on soil and the accumulation of heavy metals in our food chain are some of the concerns (Becerra-Castro et al. 2014; Elgallal et al. 2016; Jaramillo and Restrepo 2017; Khan et al. 2022; Zafar et al. 2019). Heavy metals tend to accumulate on the soil surface due to their limited solubility and absorption by plants. The presence of heavy metals in plants depends on the type of plant, soil characteristics, temperature, pH, humidity, and organic matter content (Kim et al. 2015; Dickin et al. 2016; Kidwai et al. 2022). A study by Qureshi et al. (2016) showed that heavy metal uptake is lower in food crops and roots, but higher in leafy vegetables. Arsenic (As) is one of the toxic and carcinogenic heavy metals where inhalation and/or ingestion can pose cancer risks. Locally available natural absorbents are considered economical and environmentally friendly options for treating arsenic in water (Asere et al. 2019; Neisan et al. 2023). The occurrence of soil-transmitted helminth eggs in TWW used for crop irrigation has become a public health concern in most developing countries, including Ghana, Morocco, Tunisia, and South Africa (Amoah et al. 2016, 2018; Faouzi et al. 2023; Ayed et al. 2009). Mitigating the health risks associated with helminth eggs involves wastewater application techniques, restricting crops, post-harvest handling practices, and food preparation methods (Keraita et al. 2014; Scheierling et al. 2011).

High-rate anaerobic reactors, such as Upflow Anaerobic Sludge Blanket (UASB) reactors and their modified versions, have been widely used in developing countries like India, Brazil, Vietnam, Colombia, Mexico, Egypt, and Ghana for treating municipal wastewater (Chernicharo et al. 2015; von Sperling and Cherinicharo 2005; Amaral et al. 2019; Nada et al. 2011; Arthur et al. 2022). The anaerobic wastewater treatment approach is popular due to its unique characteristics, including economic feasibility, compactness, low electricity demand, low sludge production, and reduced installation, operation, and maintenance costs (Moussavi et al. 2010; Singh et al. 2015; Capodaglio et al. 2017). In developing countries, a cost-effective and straightforward option for treating domestic wastewater is using UASB reactors combined with trickling filter systems. In Africa, two studies have evaluated the performance of full-scale UASB reactors with trickling filters for treating domestic wastewater. According to Nada et al. (2011), the scheme achieved satisfactory removal efficiencies for COD (83%), BOD (90%), and TSS (87%) with corresponding residual values of 127 mg/L, 28 mg/L and 35 mg/L for COD, BOD₅ and TSS, in Egypt. Similar WWTP configurations provided higher removal efficiencies in Ghana for COD (93%), BOD₅ (98%), and TSS (93%) but with higher residual values of 152 mg/L, 33 mg/L and 72 mg/L for COD, BOD₅ and TSS respectively (Arthur et al. 2022). Other post-treatment technologies showed promising results after UASB reactor for wastewater reclamation and reuse in arid and semi-arid regions, such as sequential batch reactor (Moawad et al. 2009), subsurface flow wetland (El-Khateeb and El-Gohary 2003).

In tropical conditions, the reactor has shown a BOD reduction of 80–90% when treating domestic wastewater (Abdel-Halim et al. 2009). However, post-treatment options are necessary to improve the quality of effluent to meet irrigation standards. Conventional processes such as maturation ponds, waste stabilization ponds, polishing ponds, constructed wetlands, rotating biological contactors, moving bed biofilm reactors, downflow hanging sponges (DHS) (Tawfik et al. 2003), and advanced oxidative processes (Daud et al. 2018) are used for this purpose.

A trickling filter is also a post-treatment option that can use different materials as biofilm support media. For example, expanded polystyrene can be an alternative material to be used in sewage treatment (Filho et al. 2023). The use of low-cost and readily available adsorbent material could be an effective option for peri-urban wastewater treatment in developing countries such as using maize cob and date palm fiber as biofilm support media in an integrated system of cascade cum trickling filter with multilayer adsorption (CCTF-MLA) showed steady removal of COD (83.4%), TN (80%) and TP (83%) (Kanwar et al 2019, 2021). A multilayer adsorption system was constructed using recycled crushed brick + rice husk + steel slags for effective removal of non-biodegradable contaminants. The removal efficiency of CCTF with a conventional trickling filter showed that the CCTF with maize cob media was the most efficient, with 91.8% BOD removal efficiency and the capacity to treat high BOD loads (Kanwar et al. 2021). Another study evaluated the removal efficiency of the locally designed trickling filter using maize cop (TF1) and date palm fiber (TF2) as biofilm support media, the removal efficiency of TF1 was observed to be 8–15% higher than that of TF2 for removing BOD, COD, TDS, TSS, EC, TN, TP, sulfates and pathogen indicators (Kanwar et al. 2021).

Little attention has been given to evaluating reuse potential of effluent from the UASB reactor combined with a trickling filter for agriculture. Many studies have been conducted on UASB reactors, but there is a lack of research on the potential of reusing effluent for agriculture. In addition, previous studies on evaluating the performance of full-scale UASB reactors combined with trickling filters have also been limited in Africa. Considering additional parameters such as nitrate, helminth egg and heavy metals in the operation guiding parameters is a new insight while adopting an integrated UASB reactor with TF in the presence of sludge recirculation from a secondary clarifier.

In the present study, we evaluated the performance of Kality domestic wastewater treatment plant that have combined UASB-TF system. In addition, the study examined the concentrations of heavy metals and pathogenic microorganisms and evaluated the reusability of effluent for agriculture.

Methods and materials

Study area and system configuration

The study was conducted at the Kality Wastewater Treatment Plant (WWTP) in Addis Ababa, the capital city of Ethiopia in East Africa. In 2018, a new configuration of a UASB reactor integrated with a trickling filter (TF) was constructed and commissioned to treat domestic wastewater due to the low treatment capacity of the previous waste stabilization pond. As the wastewater production has increased, the design capacity of the previous waste stabilization pond was exceeded. The authority has chosen an integrated UASB reactor with TF considering the resource recovery option and the potential land availability. The average percentage removal of Kality wastewater stabilization ponds was 83.6% of BOD₅, 77.6% of COD, and 66.5% of TSS with corresponding residual effluent concentrations of 47.5 mg/L, 131.4 mg/L and 71 mg/L. In addition, the concentration of NH₄+–N, NO₃–N, and TN in the effluent was 16.2, 0.3, 37.3, and 10.2 mg/L. The total coliforms and fecal coliforms removal efficiency was 99% (Belachew 2011).

The Kality WWTP is the first UASB reactor with a trickling filter in Ethiopia. The plant was operating in steady state condition during the monitoring period; the raw sewage inflow to the plant ranged from 61,080 to 70,872 m³/d, with an average flow of 65,245 ± 2729 m³/d. The plant’s treatment processes include a preliminary treatment unit operation consisting of screening, de-gritting, and degreasing units followed by a biological treatment unit process including UASB reactors and trickling filters with a secondary clarifier (TF/SC) as shown in Fig. 1. The UASB unit contains a total of 20 UASB reactor cells grouped in four lines. The disinfection process after the secondary clarifier involves chlorination and dichlorination. Excess sludge from the TF/SC system is returned to the UASB reactor. The sludge recirculation rate varies from 1080 to 1416 m³/day with an average rate of 1250.6 m³//day. The plant also has a sludge treatment line. Biogas generated in the UASBs is collected in the gas hoods and channelled to a biogas flaring unit. During the study period, a total of 15 UASBs of the 1st, 2nd, and 3rd units have been put into operation. Table 1 presents the design influent and effluent characteristics of Kality WWTP.

Fig. 1

Flow Chart of UASB reactor with trickling filter at kality WTP and sampling locations

Table 1 Influent and effluent design.

The raw sewage characteristics of the Kality WWTP with design consideration to fulfil the expected effluent quality are presented in Table 1. The (BOD:COD ratio = 470/940 = 0.5) has considered the baseline BOD and COD concentration of raw domestic wastewater for Addis Ababa as a design consideration. In addition, in our study, we have obtained the value of BOD: COD ratio ranging between 0.52 and 0.7. As reported by Kumar et al. (2010) and Wei et al. (2023), the BOD:COD ratio higher than 0.5 is considered readily biodegradable.

The UASB reactor’s hydraulic retention time (HRT) is directly linked to the speed of the anaerobic digestion process, which depends on the reactor’s size.

The UASB reactor at Kality has an HRT of 12.2 h, with an average daily sewage flow of 75,000 m³/day and an organic loading rate of 2.29 kg COD/m³ d. The Upflow Liquid Velocity of the reactor is 0.4 m/h. The plant has 20 UASB reactors divided into 4 blocks, each with a cross-sectional area of 400m² and a height of 4.9 m. The plant’s designed removal efficiencies for BOD₅, COD, and TSS are 55%, 55%, and 70%, respectively. During the study period, 15 UASBs from the 1st, 2nd, and 3rd blocks were put into operation. The operational conditions of UASB reactor and TF of Kality WWTP presented in Table 2.

Table 2 Operational conditions of UASB reactor and trickling filter

The influent flow to the trickling filters comes from the UASB treatment stage by gravity and also includes assumed 30% returning flow from the sludge stream. The trickling filter (TF) with the secondary clarifier has a BOD loading rate of 0.86 kg/m³ d and a hydraulic application rate of 0.63 m³/m² h without recirculation and 0.82 m³/m² h with circulation. The final effluent BOD concentration is expected to be below 30 mg/L, with about 90% removal efficiency. The required total filter area and the total packing volume are 5150 m² and 25,131 m³, respectively. Per trickling filter, the area is 1288 m², and the treatment volume is 6283 m³. Consequently, the filter diameter is sized to 40.5 m with packing media height of 4.88 m.

Research design

This study monitored compliance by following a special sampling and analysis program that was more stringent than the routine program. Heavy metals, such as Zinc, Nickel, Lead, Chromium, Cadmium, Copper, and Iron, and microbiological contaminants, including helminth eggs and E. coli, were continuously monitored for 30 days (May 11–June 09), during the dry season in 2021 besides routine operational parameters. Wastewater samples were collected from six different locations as indicated in Fig. 1, including raw sewage, injection point of recycled excess sludge at the outlet of the preliminary treatment unit, UASB rector outlet, TF outlet, secondary clarifier outlet, effluent after disinfection. For each location, a single composite sample was prepared and taken for laboratory testing. The parameters analyzed included Chemical oxygen demand (COD), Biological oxygen demand (BOD), total suspended solids (TSS), Volatile suspended solids (VSS), total nitrogen, Nitrate (NO₃–), Nitrite (NO₂–), Ammonium–nitrogen (NH₄–N), total phosphorus, and Sulphate (SO₄²⁺) and E. coli. In addition, the final effluent was examined for agricultural reuse considering the concentration of heavy metals including (Zinc (Zn), Nickel (Ni), Lead (Pb), Chromium (Cr), Cadmium (Cd), Copper (Cu) and Iron (Fe) and helminth eggs count.

Sampling and analytical procedures

Samples were collected for 30 days from May 11, 2021 to June 9, 2021 May and analyzed during the dry-weather period. There are 6 sampling locations and indicated in Fig. 1. A total of 150 composite samples were sent for laboratory tests. In addition, a grab sample was taken daily for microbial analysis at the outlet after the disinfection unit. A sampling method developed by Plosz et al. (2010) was used to represent the daily variation of wastewater’s physio-chemical properties. Initially, individual samples were collected every 2 h from sample locations 2, 3, and 4 due to the absence of an automatic sampler and stored in a refrigerator. Composite samples were then created by uniformly mixing the individual samples. The flow-proportional composite samples were taken every 2 h for 24 h. The grab portions of each sample required per unit of wastewater flow were determined using Eq. 1.

$$ {\text{Multiplier}} = { }\frac{{\text{Voume of composite sample desired}}}{{{\text{Average flow rate}} \times {\text{number of portion}}}} $$

(1)

The APHA (2012) sample handling techniques were applied to preserve the collected samples until they were tested in the laboratory. The collected test and associated composite samples were stored at 4 °C in an ice box. Each sample was marked with a date, location, and parameters to be tested, and then submitted for laboratory analysis.

The laboratory analysis of water and wastewater was conducted using the Standard Method developed by the American Public Health Association (APHA) (Greenberg et al. 1992). Duplicate samples were tested. In-situ testing of pH, conductivity, TDS, and temperature of the wastewater samples was performed using a handheld multiparameter test instrument (Model 99720, Taiwan). The BOD of the samples was determined by the BODTrak II Respirometric Hach method (BODTrak II Apparatus ~ 473 mL/4 pcs, 5 days, 24 V, UL CSA). The TSS was analyzed using APHA methods. The total hardness (TH) of the samples was tested using complexometric titration methods, with Eriochrom Black T used as a complexometric indicator. For the testing of sulfate (SO₄²⁻), the Hach approach of the Sulfaver 4 method was used with a spectrophotometer (Hach DR 6000, UV VIS (190–1100 nm), Colorado, USA). The nitrogen (ammonia) was analyzed using the Phenate Spectrophotometric method, whereas the nitrate (NO₃–) was analyzed using a phenol disulphonic acid (PDA) method and read by a UV–Vis Spectrophotometer (UV 2600 Shimadzu, Japan). To analyze the volatile suspended solids (VSS) in the samples, gravimetric analysis methods were used. The E. coli content of the samples was enumerated using a compact dry plate method, and a Priya Moodley, Sedimentation and suspension/flotation method of microbial analysis was adapted for Helminth Eggs examination in this study, as explained in (Jaromin-gleń et al. 2017). Lastly, the heavy metals content of the sampled wastewater was analyzed by atomic absorption spectroscopy (AAS, Jenna, Germany). The Shapiro–Wilk test was conducted to identify the distribution of raw sewage flow and three monitoring wastewater parameters, namely BOD₅, COD, and TSS. This is necessary to select the appropriate statistical method for analysis (Mishra et al. 2019).

Sample characteristics

The Shapiro–Wilk test has indicated that the raw sewage flow is normal (W = 0.94, p-value = 0.104). For BOD₅ and COD in five sampling locations, a visual examination of the histogram of X and the QQ plot confirmed that the data were approximately normally distributed, and a parametric test was chosen for analysis. The mean and standard deviation were used to summarize the data. As the TSS samples at station 5 had a non-normal distribution, a non-parametric test was used, and the median with an interquartile range was used to summarize the TSS. The Pearson product correlation of BOD₅ and COD of raw sewage was found to be strongly positive and statistically significant (r = 0.886, p < 0.001). An increase in BOD₅ would lead to a higher COD in the sewage. Similarly, the linear relationship between COD and TSS was found to be moderately positive and statistically significant (r = 0.593, p < 0.01). An increase in COD would lead to a higher TSS value of sewage. The flow of raw sewage was found to have a moderately negative relationship with COD and TSS parameters. An increase in flow would lead to a lesser COD and TSS parameter. The increased flow from various sources of the uncontrolled sewer lines might result in dilution which causes a reduced COD and TSS concentration.

In addition, the TSS parameter has shown a moderately positive relationship with BOD₅. An increase in the TSS parameter of the raw sewage might lead to a higher BOD₅ parameter. Lastly, the COD to VSS ratio of the sewage sample was found to decrease, which is consistent with the COD to VSS ratio of 1.44 reported for domestic wastewater characteristics (Ahnert et al. 2021). In the dry season, the higher COD-containing effluent is associated with the volatile suspended units accounted for by the TSS. The high inflow condition has created higher turbulences that result in the pushing of the floatable solids into an effluent stream and reduces the sample TSS, which is not accounted for in the COD load. The volatile suspended solid contains a higher concentration of microbial organisms and associated organic matter, which can be accounted as a main cause of increased BOD₅ (Abdulla et al. 2012).

Results and discussion

Influent flow rate and wastewater characteristics

The characteristics of raw sewage are vital in evaluating the removal efficiency of any treatment technology. The flow and physiochemical characteristics of raw sewage were measured during the study. The average daily sewage flow was 68,783.3 ± 0.04 m³/day, there was no significant fluctuation in the volume that could affect the treatment process in the facility. The BOD₅ and COD variation follows similar patterns (Fig. 2) with average concentrations of 350 ± 76 mg/L and 594 ± 117 mg/L, respectively, which compared well with medium-strength sewage (Gaur et al. 2017). However, the sewage concentration was very low compared to raw sewage of similar treatment configurations (Nada et al. 2011; Ahmed et al. 2018). In addition, the raw sewage BOD:COD ratio was found to be between 0.52 and 0.71 with an average value of 0.59. According to Bader et al. (2022) if BOD:COD > 0.6, the waste is fairly biodegradable, and can be effectively treated biologically. As reported by Kumar et al. (2010) and Wei et al. (2023), the BOD:COD ratio higher than 0.5 is considered readily biodegradable.

Fig. 2

Variation of Raw Sewage Parameters

In Addis Ababa city, the sewer infrastructure management is poor and there is a gap in legislation and its enforcement (Ali and Robele 2023). In addition, the Addis Ababa Water and Sewerage Authority reported illegal connections from factories and industries affecting the performance of WWTP. Similarly, a previous study showed that industries in Addis Ababa discharge their untreated wastewater to nearby drainage systems (Seyoum et al. 2017). Significant variations of BOD₅ and COD of domestic wastewater may result in effects such as shock loads and affect microbial activities of a given biological wastewater treatment (Metcalf & Eddy 2003; Leitão 2004). Variations of sewage temperature and pH were relatively small with fluctuations in the range of 22 °C–24 °C and 7.0–7.5, respectively. These sewage temperature and pH ranges are favorable for optimal reactor performance (Leitão et al. 2005; Kaviyarasan 2014).

Removal efficiency of UASB reactor

The performance of the UASB reactor was computed based on influent and effluent sewage characteristics. The performance of the UASB reactors presented by COD, BOD₅, and TSS percentage removal values is depicted in Fig. 3A, B. During the monitoring period, the OLR was 1.5 ± 0.3 kg COD/m³ d which is lower than the design value 2.29 kg COD/m³ d achieving 66% ± 10.6%, 73.4% ± 7.85% and 63.4% ± 11.8%, respectively for COD, BOD and TSS. The percentage removal of COD and BOD in the UASB reactor is enhanced by the residual organic substrates and nutrients from the recirculated sludge from the secondary clarifier. The average nitrate concentration of raw sewage was 6.3 mg/L which was injected by 81.3 mg/L through sludge recirculation from a secondary clarifier that enhanced the bacterial processes of the reactor.

Fig. 3

Removal efficiency of UASB reactor (A) BOD % removal and COD % removal with design percentage removal (B) TSS % removal and design percentage removal

The fluctuation in OLRs may lead to operational issues such as pH fluctuations and accumulation of toxic intermediates. Similar performance was reported with a removal rate of 70% ± 11.2, 70.35 ± 13 and 85.56 ± 7.2 for COD, BOD_5, and TSS, respectively, when the UASB reactor performed under a maximum OLR of 2.2 kg COD/m³ d (Nada et al. 2011). The sludge recirculation helps maintain a healthy and active microbial population within the reactor. It also helps stabilize treatment performance by providing a source of acclimated microorganisms capable of handling fluctuations in organic load and wastewater characteristics (Aslanzadeh et al. 2013). A higher removal efficiency as high as 90% for BOD₅ with corresponding COD and TSS removal rates of 77% and 73%, respectively, reported when the reactor performed with OLR ranging from 1.3 ± 0.79 kg COD/m³ d with longer HRT of 45.77 ± 24 (Arthur et al. 2022).

UASB reactors found in similar climatic conditions might face operational problems as a result of OLRs fluctuations. Because rapid fluctuations or sustained high OLRs can disrupt the microbial community structure and function, leading to decreased organic matter removal efficiency and instability in reactor performance.

This study obtained a lower percentage removal of TSS than reported by similar literature which could be attributed to poor sludge blanket characteristics such as density, settling velocity and biomass concentration (Rizvi et al. 2017). Kaviyarasan (2014) reported that domestic sewage treatment using a UASB reactor without a primary treatment unit resulted in poor removal of macronutrients and TSS, caused by the rapid rise in sludge bed height and frequent sludge removal. Hence, it increases the disturbance to microbial growth. This study has comparable results though concentrations of the influent are lower. COD, BOD₅ and TSS removal was 51 ± 13, 56 ± 11 and 54 ± 13%, respectively (Gaur et al. 2017).

Temperature plays a key role in the anaerobic process of microorganisms in the UASB reactor. In our study, the average operational temperature of the reactor was 22.3 ± 0.5 °C which is favorable for good reactor performance both in pilot scale and full-scale reactors with a removal rate ranging from 79–81% for COD and 77–83% for BOD₅ (Ahmed et al. 2018; Leitão et al. 2005; Lew et al. 2011; Kaviyarasan 2014; Rizvi et al. 2017; Zhang et al. 2018). Apart from climatic conditions, wastewater management practices and sewer infrastructure conditions also affect the efficiency of the UASB reactor when used as an end-pipe treatment system (Gaur et al. 2017). This is a main concern in sub-tropical countries, including Ethiopia, where sewer systems are not fully controlled, maintained, and well-regulated (Kennedy-Walker et al. 2020). Additionally, the Addis Ababa Water and Sewerage Authority has no regulation to control the sewage characteristics from various sources. This could explain the probable operational challenges encountered by the UASB reactor and the associated removal efficiency.

The reactor has achieved a maximum nitrate removal efficiency of 63% as depicted in Fig. 4. However, considerable fluctuation was also observed which might be attributed to poorly managed USAB reactor operation. The release of more nitrate from the reactor might indicate the introduction of aerobic or semi-aerobic conditions in the UASB reactor. Similar studies reported an average removal rate as high as 80% of nitrate (Ahmed et al. 2018) and as low as 0.1% removal efficiency (Arthur et al. 2022).

Fig. 4

Nitrate removal efficiency of UASB reactor

Removal efficiency of trickling filter (TF)

The COD and BOD₅ removal efficiencies of the trickling filter with and without secondary clarifier are presented in Figs. 5A, B and 6A. Most studies reported the combined performance of the trickling filter with a secondary clarifier. However, our study evaluated the performance of TF and TF with SC separately to assess the removal efficiency of each unit. Post-treatment with TF achieved an average of 23.3% for BOD, 28.9% for COD, and 31.9% for TSS removal efficiency. An increased concentration of BOD and TSS (negative removal efficiency) was reported by a similar study while 4% COD removal efficiency was achieved by the TF (Arthur et al. 2022). On the other hand, a higher BOD removal efficiency of 47% was achieved while COD removal efficiency was only 2% (Ahmed et al. 2018). The TF in our study has good performance compared to similar studies. The combined treatment from TF and SC enhanced the percentage removal of 40.8 ± 13.7, 34.4 ± 14.8 and 45.4 ± 15.7 for COD, BOD₅, and TSS, respectively. The results showed fluctuation indicating unstable performance of the trickling filter and secondary clarifier. The post-treatment performance is satisfactory compared to similar post-treatment having percentage removal ranging from 48 to 57 for COD and 59 to 68for BOD (Awuah and Abrokwa 2008; Ahmed et al. 2018). A recent study in Ghana reported higher performance of trickling filters with a final settling tank as post-treatment activities after the UASB reactor—69.3% for COD, 72.5% for BOD₅, and 72.5% for TSS (Arthur et al. 2022). The study also obtained higher performance in nutrient removal in the UASB reactor.

Fig. 5

Removal efficiency of TF and SC (A) BOD % removal of TF and TF with SC (B) COD % removal of TF and TF with SC

Fig. 6

Removal efficiency of TF and TF with SC (A) TSS % removal of TF and TF with SC (B) concentration of TSS, nitrite and its design value at SC outlet

Figure 6B shows the incomplete nitrification process that affects the Nitrite concentration of the effluent at the secondary clarifier, which might cause variable and smaller NH₄–N removal efficiency. The sudden drop in TSS was related to sludge discharging. The trickling filter achieved NH₄–N removal efficiency with higher variability, ranging from 6.20 to 79.10%. The Nitrite (NO₂–N) levels exceeding 0.8 mg/L (design value) in the secondary clarifier indicate excessive biomass growth or solids accumulation in the filter, causing higher TSS values of the effluent. A similar study reported that the operation of secondary clarifiers could affect the trickling filter performance through nitrite production and algal biomass growth, which tends to increase the TSS value of the trickling filter (Arthur et al. 2022).

The nutrient removal efficiency of the TN, TP and NH₃– observed at the post-treatment unit presented in Fig. 7A, B. The TF achieved an average of 24.3%, 11.8%, and 46% removal efficiency of TN, TP and NH₃–N respectively. Higher performance was observed for NH₃–N with 60.9% removal of TF and 87% removal of TF with a sedimentation tank (Ahmed et al. 2018). In our study, the TF with SC improved the NH₃–N removal efficiency to an average of 62.5%. On the contrary, a lower removal efficiency of 27.4% was reported in a similar study (Arthur et al. 2022). The considerable fluctuation in the NH₃–N removal efficiency of the TF and the combined TF with SC requires careful monitoring of influent characteristics, operational parameters, and system performance to optimize treatment efficiency and ensure consistent NH₄–N removal in TF and TF-SC systems.

Fig. 7

Performance efficiency of TF and SC (A) percentage removal of TN and TP of TF (B) percentage removal of NH3–N of TF and TF with SC

Overall efficiency of Kality WWTP

COD, BOD and TSS removal efficiency

The performance of Kality WWTP was computed based on raw sewage and final effluent concentration after disinfection. The total COD and BOD₅ removal efficiency of the plant and the corresponding effluent quality are illustrated in Fig. 8A, B. The WWTP achieved a percentage removal of 80.5 ± 5.3 for COD and 82.9 ± 5.6 for BOD₅. Figure 9 presents the TSS removal efficiency and the corresponding effluent quality of the scheme with an average percentage removal rate of 80.9 ± 6.4. Various studies on UASB-TF reported a higher percentage removal efficiencies ranging from 83–94, 90–98.8 and 87–92, respectively, for COD, BOD₅ and TSS (Ahmed et al. 2018; Arthur et al. 2022; Nada et al. 2011; Awuah and Abrokwa 2008). The decreased performance of the scheme at Kality is potentially related to unstable processes in the UASB reactor, trickling filter, and secondary clarifier. During the monitoring period, the BOD₅, and TSS concentration of the effluent exceeded the design value of 35 mg/L (for agriculture reuse) with an average value of 61.7 mg/L for BOD₅ ranging from 27.5 to 80.7 and 85.8 mg/L for TSS ranging from 40 to 240 mg/L. The higher TSS value at the outlet of the secondary clarifier indicated a lower removal efficiency of NH₄–N. TSS interfered with the biomass’s settling process and reduced the treatment's overall effectiveness.

Fig. 8

Overall removal efficiency of kality WWTP (A) Overall BOD₅% removal (B) overall COD % removal

Fig. 9

TSS Removal performance of kality WWTP

The average COD concentration of the effluent achieved a 125 mg/L value ranging from 75.5 to 201.5 mg/L which is higher than the discharge limit of 100 mg/L for unrestricted reuse. The statistical analysis revealed that the concentration of both BOD₅ and COD in the effluent were significantly higher than the design value (t (29) = 10.244, p < 0.01) and (t (29) = 4.445, p < 0.01), respectively. The Kality WWTP operates at an ambient temperature of 22 to 24 °C, which is highly suitable for microbial activities. As summarised in Table 3, the overall efficiencies of the UASB/TF at Kality for COD (80.5%), BOD₅ (82.9%), and TSS (80.9%) are slightly lower than the efficiencies reported by other studies done on full scale UASB-TFreactors (Arthur et al. 2022: Nada et al. 2011; Ahmed et al. 2018; Awuah and Abrokwa 2008).

Table 3 Domestic wastewater treatment experiences of the UASB reactors

Overall, based on the observed concentrations, the effluent quality from Kality WWTP exceeds the requirements set for unrestricted agricultural reuse but meets the national limits for discharging into inland waterbodies.

Nitrite and nitrate nitrogen and sulphate removal efficiency

The nutrient removal efficiency of the plant was evaluated by comparing the concentration of nitrite, nitrate, and ammonium in the form of nitrogen in the raw sewage with the effluent. NO₃–N in irrigation water enables crops to meet their N requirements and minimize their dependence on the costly commercial fertilizer. In this study, the concentration of NO₂–N and NO₃–N increased in the effluent with 90.9% and 92.2%, respectively (Fig. 10A, B), which might be due to nutrient overloading. The average concentration of NO₃–N in the effluent was 81 mg/L which is > 30 mg/L and this imposes a severe degree of restriction for reuse (FAO 1989). This elevated level of nitrate poses a severe restriction on the reuse of the effluent due to potential risks to human health and the environment. Nitrate pollution in water bodies can lead to contamination of drinking water sources, eutrophication, and adverse effects on aquatic ecosystems. Therefore, mitigation measures such as improving wastewater treatment processes or implementing additional treatment steps are necessary to reduce nitrate levels in the effluent to meet safety standards for reuse or discharge.

Fig. 10

Nitrite and nitrate removal performance of kality WWTP (A) nitrite % increase, influent and effluent concentration (B) nitrate % increase, influent and effluent concentration

According to the U.S. Environmental Protection Agency, consumption of water containing nitrate–N above 10 mg/L may cause illness. The higher concentration of nitrate observed in our study could be linked to the operational instability of the UASB reactor, trickling filter, and secondary clarifier. The amount of recirculating sludge from the secondary clarifier back to the UASB reactor needs to be optimized to improve the operational stability of the wastewater plant.

The average percentage removal of NH₄–N and SO₄⁻² in the scheme was 93.9 and 37.9, respectively with corresponding average residual concentrations of 0.05 mg/L and 245.6 mg/L (5.1 me/L). The result indicated that the concentration of NH₄–N and SO₄⁻² fall within the usual range in irrigation water as shown in Fig. 11A, B. Results from similar configurations reported an efficiency of − 37.7% (increased concentration in the effluent) and 43.7 for NO₃–N and SO₄⁻², respectively (Arthur et al. 2022).

Fig. 11

Sulphate and ammonia nitrogen removal performance of kality WWTP (A) influent, effluent and percentage removal of sulphate (B) influent, effluent and percentage removal of ammonia nitrogen

Microbiological removal efficiency

Microbiological parameters i.e. Helminthes eggs and E. coli of the effluent were used to evaluate the performance of the disinfection unit. The result indicates that the effluent is free from E-coli but the Helminthes eggs count ranged from 5 to 11 which exceeds the limit set for irrigation reuse, i.e. 1viable/L (WHO 2006; AAWSA 2017; Jimenez et al. 2017). One-sample T-test analysis showed that the Helminthes egg amount in the effluent was significantly higher than the design value (t (8) = 8.632, p < 0.05). Helminth eggs can survive in water, soil and crops for several months as reported by (Calderon-Roca 2017) and difficult to inactivate in wastewater treatment plant (Jimenez et al. 2017). Moreover, empirical evidence has found that crops irrigated with treated wastewater contained high levels of helminth eggs (Hajjami et al. 2013). Therefore, the unrestricted use of effluent from Kality WWTP for irrigation could be a threat to public health.

Sodium adsorption ratio (SAR) and other ions

The sodium adsorption ratio (SAR) is important for determining the suitability of irrigation water for use on land. It can be computed using sodium, calcium, and magnesium concentrations in the irrigation water or soil (Oster et al. 2016). The infiltration rate of water into the soil is measured by the combined SAR and water salinity (EC, electricity conductivity) of effluent. In our study, the observed average SAR value of < 1 mg/L and EC = 0.98dS/m lead to slight to moderate restriction for irrigation reuse (FAO 1989). Higher electrical conductivity can negatively impact the availability of water to plants and reduce crop growth and yield. Table 4 shows the measured mean values of effluent cations/anions and heavy metals and the usual range in irrigation water. The results indicate that the mean calcium, sodium, sulfate and ammonium–nitrogen concentrations are within the usual range of irrigation water. However, the mean magnesium, potassium and nitrate–nitrogen levels are high and may cause potential irrigation problems due to soil salinity, higher pH and toxicity. High levels of nitrate–nitrogen can reduce growth and interfere with the uptake of nutrients like calcium, magnesium, and potassium.

Table 4 Treated wastewater quality mean value compared to FAO irrigation water quality standard

In our study, it was found that all trace elements except chromium were within the permissible limit. However, the concentration of chromium in the samples ranged from 1.9 to 4.44 mg/L, which exceeds the maximum permissible limit. Additionally, empirical evidence has shown that the accumulation of chromium in soil, with a mean value of 4.2 mg/L, results in the concentration of chromium in grain that exceeds the dietary limit (Hassen et al. 2013). The indiscriminate disposal of wastewater in the sewer network might intrude heavy metals in the raw sewage conveyed to Kality WWTP and adversely affect its removal efficiencies. Studies have shown that elevated levels of Cr to 1.3 mg/L can hinder the biochemical reactions of anaerobic microorganisms, leading to a significant decrease in the removal efficiency of total suspended solids (TSS), chemical oxygen demand (COD), and biological oxygen demand (BOD₅) (Abou-Elela et al. 2018). It is, therefore, important to prevent discharges of wastewaters that have heavy metals and other toxic substances into domestic sewerage system.

Conclusion

The purpose of this study was to evaluate the efficiency of an integrated/TF reactor and secondary clarifier in treating municipal wastewater. The study also looked into the agricultural reuse potential of the treated effluent. Our study concluded that the effluent from the Kality WWTP meets the local inland discharge standards. However, direct reuse of the UASB/TF system effluent for crop production has health risks associated with the above-limit concentrations of chromium, magnesium, nitrate, potassium, and helminth eggs exceed the acceptable limits. Use of appropriate tertiary treatment processes are required to ensure safe reuse of the treated effluent for irrigation. We recommend regular monitoring of heavy metals, magnesium, potassium, nitrate, and helminth egg counts, especially in urban areas with poor sewerage system management.

It is important to note that this study focused on the dry weather performance of the treatment plant in treating municipal wastewater that comes from domestic and non-domestic sources. Its findings on the characteristics of the sewage and treatment units’ performance efficiencies can serve as a good source of information for future wastewater related studies in areas having similar operational contexts. Future research should cover a larger sampling scope, spanning both dry and wet seasons, for a better understanding of the UASB/TF system’s performance across varying conditions. The wastewater treatment needs to have further treatment facilities such as constructed wetlands or polishing ponds. The breakeven point of existing chlorination disinfection steps should be kept seriously to protect the further generation of nitrate into the effluent emitted to the environment.