Enhancement of overloaded waste stabilization ponds using different pretreatment technologies: a comparative study from Namibia

Waste stabilization ponds (WSP) are a well-established wastewater treatment technology in Namibia. But they are often overloaded and we still lack concepts and technologies for improvement. Therefore, this study presents the full-scale implementation of two pretreatment technologies to reduce the in ﬂ ow of organic and solid loads into a facultative pond. We speci ﬁ cally compared the effects of anaerobic biological and mechanical pretreatment by an upstream anaerobic sludge blanket (UASB) reactor and a 250 μ m micro sieve (MS). Not only in Namibia but also in most sub-Saharan countries, there is little experience with these technologies for the treatment of municipal wastewater in small and fast-growing local communities. Both technologies were tested in parallel for a period of 17 months and proved operational. While the UASB achieved better removal results with respect to chemical oxygen demand (COD) and suspended solids (TSS), the MS was more ﬂ exible in handling changing in ﬂ ow patterns and had a much smaller footprint. The average total COD reductions of the MS and the UASB were 22 and 50%, respectively. TSS were removed by 45% with the MS and by 57% with the UASB reactor. Therefore, UASB and MS are viable options for the enhancement of existing WSP to reach better ef ﬂ uent values of the facultative pond.

especially for hygienic parameters but also for COD removal. Not only the increasing population and the related need for more irrigation water with valuable nutrients but also the reduction of greenhouse gases impose a growing burden on WSP (Shelef & Azov ; Hernandez-Paniagua et al. ). A typical strategy for capacity enhancement is volume extension (number and or size of the ponds) which is accompanied by growing land requirements.
Other options may present themselves through upgrades of existing WSP with more advanced treatment technologies, such as anaerobic biological or mechanical treatment units.
This study compares an upstream anaerobic sludge blanket (UASB) reactor to a micro sieve (MS) as potential pretreatment technologies in order to reduce the load of organic carbon (measured as chemical oxygen demand, COD) and suspended solids entering an existing overloaded WSP in Northern Namibia. The potential of these technologies to reduce COD and solids has been reported by Lazarova & Bahri () with a wide range of values of 40-70% and 20-75% for UASB and MS, respectively.
Also, the effect of such measures on the performance of consecutive ponds has not been monitored in depth in any sub-Saharan countries.
Due to high COD and TS loads into primary facultative ponds, sludge accumulation is also an issue and reduces the treatment capacity. A typical WSP set-up would include an anaerobic pond to reduce COD and TSS loads into the facultative pond (Shilton ; Sperling ). However, this would not reduce potential methane emissions and would not solve the problem of the quickly accumulating sludge.
In order to address this, UASB reactors have already been In Europe, Jahn et al. () and Walder et al. () researched an MS within the Austrian context. In Namibia, micro sieves have so far been implemented as pretreatment for an industrial wastewater treatment plant followed by membrane bio-reactors (Prösl et al. ) and as post-treatment for a municipal wastewater treatment plant (Müller ). An MS for the pretreatment before a WSP, however, has been implemented for the first time in this research. This is the first study which compares an UASB reactor (biological pretreatment) with an MS (mechanical pretreatment) to relieve overloaded WSP, with Namibia as an example. The results are especially important for fast-growing communities in warm climates with the need of water reuse for irrigation and for regions without perennial streams (Butler et al. ).

WSP and pretreatment technologies
This project was conducted at an existing WSP system in a regional capital in Northern Namibia. The original WSP system as planned in 2004, consisted of two parallel lines  Figure S1). According to the design, the feed rhythm alternated between the two lines.
The two facultative ponds have the largest volume of 16,000 m 3 each and a surface of 11,000 m 2 . Both are followed by three maturation ponds with smaller surfaces and volumes. The total water surface of the WSP is 40,500 m 2 with a total volume of 53,000 m 3 . The water level decreases from 1.5 m in the first pond to 1.1 m in the last pond (Supplementary Table S1). Due to a high groundwater table and a nearby ephemeral stream, the earth dams constructed above the natural surface are covered with concrete, while the ground of the ponds is lined with clay.
Two pretreatment technologies were tested: an MS and an UASB reactor. Upfront, a coarse screen with 0.01 m bar spacing followed by a 56 m 3 open collection chamber to buffer peak inflows was installed ( Figure 1). The pretreatment technologies for the upgrade were installed in line A. The UASB reactor had a volume of 42 m 3 and was continuously fed with 3-4 m 3 /h inflow of raw wastewater, resulting in a hydraulic retention time of 10-14 h. In parallel, an MS (Noggerath Rotary Drum Screen RSJ-MG ® ) with a 250-μm monodur polyamide mesh was installed. Its drum diameter was 1.6 m, with a length of 1.5 m, resulting in a total area of 7.5 m 2 . Aside from the standard monitoring, a 2-week intensive testing phase was carried out from day 265 to day 275. During this period, different flows and operational settings were applied. These included an increasing inflow from 28 to 60 m 3 /h (maximum capacity of the feed pump: 75 m 3 /h), an increase of the impounding depth from 10 to 18 cm and chemical cleaning with a 2% potassium hydroxide solution at 60 C.
Additionally to the pretreatment, pond A1 was renewed by emptying out the settled sludge and installing two floating

Site development in Outapi
Originally about 2,500 of 3,000 inhabitants were connected to the sewer system, and the pond system treated their wastewater. By 2018, the connected population had almost tripled up to nearly 7,000 people. However, due to the high population growth rate of 9.3% per annum, the total population in 2018 was estimated at 12,000 (Mwinga et al. ). This resulted in only 58% of the total population discharging their wastewater to the sewer and wastewater treatment system, while 69% were connected to the town's water supply system (Mwinga et al. ).
The WSP were designed with no overflow to the surrounding ephemeral watercourse, so that all water was supposed to evaporate. Already at the early stage of operation, an additional evaporation pond with a surface area of 41,000 m 2 and a volume of 20,500 m 3 (Supplementary Figure S1 and Table S1) was built with simple earth dams.
But due to higher flow rates, especially during the rainy season, the evaporation pond was overflowing regularly, posing a potential health risk to humans and grazing animals.

Inflow characteristics
In this study, wastewater characteristics were monitored extensively for a long period of time. Such detailed information has so far not been available for the sub-Saharan context. The collected data included water quality parameter as well as flow patterns.
The mean total inflow to the WSP was 802 (±177) m 3 /d during the observation period, with a peak inflow of 1,330 m 3 /d on day 267 (Supplementary Figure S2). The maximum inflow was reached after rainfall events due to surface water entering the sewer system. Compared to preceding years, there was hardly any rain in the region during summer. This is also an indicator of changing rain patterns. In comparison, previous years recorded the highest hydraulic inflow to the plant with almost 2,000 m 3 /d after heavy rainfalls.
Over the past years, the mean daily inflow has increased

Pretreatment technologies
Two different pretreatment technologies, a biological system relying on anaerobic degradation processes (UASB) and a mechanical treatment system (MS) were monitored in parallel to evaluate their potential for reducing the COD and solids load into the WSP. Besides the settling of the pCOD and the TSS in the UASB, anaerobic microorganisms also reduced sCOD, nutrients, and pathogens. In comparison, the MS used a solely mechanical sieving process to reduce pCOD and TSS. The continuous operation of the UASB required a larger buffer volume to compensate for peak inflows, while the MS was very flexible with changing flow patterns.
Over the course of the research period, the MS received on average 357 (±152) m 3 /d and the UASB 60 (±33) m 3 /d.
Changing treated volumes depended on the total inflow fluctuation and peak times as well as on the pump capacities.
Peak inflows were 235 m 3 /d for the UASB and 873 m 3 /d for the MS.

Micro sieve
The different operation phases of the MS are indicated in Over the whole study period, the tCOD removal was 22 (±18)% and pCOD removal was 28 (±24)%. Therefore, the installed MS reached good percentage removal which was within the range of others, e.g. the 15-25% tCOD removal    Another important aspect of the MS was the spray water consumption in relation to the treated wastewater flow. For a sustainable operation process, water from the MS effluent was used to spray the sieve itself instead of using valuable, high-quality tap water. Nevertheless, spray water use should be as low as possible to reduce energy consumption for the process of water pump. This study shows that the percentage of spray water did not depend on the inflow but on the impounding depth. With an impounding depth of 10 cm, there was an average need of just below 5% while with a depth of 18 cm less than 4% was needed (Figure 4(b)).
This is almost double the 2-3% measured by Walder et al.
() in Austria who have cleaned their sieve with water and a combined air injector.

UASB reactor
For the first 2 months after commissioning of the pretreatment stage, there was only a small COD reduction in the UASB reactor. Total COD was reduced by only 18% from 838 to 685 mg/L, while the sCOD remained constant at around 400 mg/L. After the inoculation of the UASB reactor with anaerobic sludge from the bottom of pond B1 on day 65, the tCOD concentration reduced over 3 months down to 234 mg/L and sCOD to 102 mg/L ( Figure 5). This improvement of the performance was partially attributed to this inoculation but also to a rise in water temperatures.
The temperature increased from 23.5 C in winter (day 1 to day 30) up to 28.5 C in summer (day 75 to day 145).
The temperature influence was also evident during later  Figure S4).

Water quality comparison
Both, UASB reactor and MS, are valuable pretreatment technologies that have been implemented in different contexts. Within the local context of this study, the direct comparison of the effluent values showed that the percentage removal of the UASB was often higher than the ones of the MS. With regards to the tCOD removal, the UASB mostly came close or above 50%, while the MS seldom achieved over 50% (Figure 7(a)). This was mainly a result of the sCOD reduction in the UASB that did not exist in the MS. For the pCOD removal, the MS delivered more values above 50% but never above 75% like the UASB did ( Figure 7(b)). The average tCOD reduction of the MS was 22 versus 50% of the UASB. TSS were removed by 45% with the MS and by 57% with the UASB reactor.
Even though the main purpose of both technologies was to reduce COD and TSS loads, they also had a visible effect on the microbiology. While the MS removed up to 50% of total coliforms and E. coli, the UASB reached even better removal efficiencies of 50-75% for E. coli and even above 75% for total coliforms (Figure 7 and 306 (±58) mg/L, respectively (Figure 8(b)). But also for the pCOD, the UASB had a better removal of 45% with an outflow of 313 (±136) mg/L compared to 28% and 431 (±128) mg/L for the MS (Supplementary Table S2).
With regard to the potential water reuse purpose for irrigation of fodder crops, nutrient removal or conservation is another important aspect to be considered. As shown in The other two main differences of this pond (A1) were the reduced inflow loads of COD and TS (due to the pretreatment) and the full available pond volume due to initial sludge removal. The total inflow into pond A1 was the combined effluent from the UASB reactor (16%) and the MS (84%), while pond B1 received 100% raw sewage.
The tCOD concentration in the effluent of pond A1 was 21% lower than that of B1 with 465 (±93) versus 588 (±123) mg/L, respectively (Figure 8(a)). Also, the pCOD concentrations were lower in A1 with 380 (±92) mg/L than in B1 with 461 (±125) mg/L. However, the sCOD concentrations with 89 (±28) mg/L for A1 and 89 (±22) mg/L for B1 were the same (Figure 8(b)). This clearly showed that the enhancement of the pond system had its main impact on the removal of the pCOD. it has to be stated that disinfection was not the main task of facultative ponds. This had to be achieved by the following maturation ponds (Shilton ; Sperling ).
In order to judge the treatment capacity of the ponds, the effluent concentrations were one aspect as they are relevant for the intended irrigation purpose. However, due to high evaporation losses, those concentrations can be misleading in terms of the functionality of the system. Therefore, we also considered loads and their reduction.
The mean tCOD loads at the outflow of pond A1 were 158 (±69) kg/d with a removal efficiency of 49% compared to the inflow into line A. At the same time, the removal after pond B1 was only 36% with an average load of 211 (±117) kg/d ( Figure 10) which can be difficult to acquire in the region. Also, further treatment of the sieving residue is required. Ideally, this is done with a fermentation process, so that the produced biogas can be transferred into power. Alternatively, the residue can be composted and after stabilization and drying used as fertilizer.
During the research period, operation and maintenance were conducted jointly by project partners, students, and the local Namibian plant operators. They were trained during the implementation of the project and are afterwards capable to sustain the required daily works. improves the approach of the operators, who might have interpreted WSP as maintenance-free. Thirdly, spray water is necessary to clean the sieve. Ideally, this is implemented through a recirculation system of process water. Water consumption could be further reduced with a combined air and water cleaning.
The reduction of COD and solid loads into the first pond was achieved by both technologies with better effluent values of the UASB. However, the MS was more flexible with changing inflows and large volumes. The UASB further reduced sCOD, pathogens, and small amounts of nutrients, which is beneficial for further treatment with ponds but not strictly required. In contrast, phosphorus and nitrogen are valuable nutrients for the projected irrigation. Little to no removal of TN and TP was observed with the pretreatment, only later small amounts were consumed by algae and therefore remained in the system and were available for further irrigation purposes.
A positive effect of sludge removal, pretreatment, and baffles in line A was evident by better effluent values from A1 than with the unimproved pond B1. A further benefit of the pretreatment will be lower sludge accumulation rates and therefore longer removal intervals. Further research will focus in detail on the effluent values of ponds A4 and B4 and the suitability of the water quality for irrigation purposes. Such research will also include the maturation ponds and especially the influence of algae on the irrigation water as well as the disinfection and reduction capacity for pathogens. Long-term performance and operation stability data of UASB and MS will also be available for this local context.