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Pharmaceutical compounds comprise a wide range of substances that are consumed in large quantities by modern societies and are generally released into local sewer networks through excretion. This research aimed to identify the factors affecting the removal efficiencies of these compounds in biological wastewater treatment plants (WWTPs) under different environmental conditions. Of the pharmaceutical compounds selected for this study, the highest influent concentrations measured in municipal wastewater treatment plants (MWWTPs) were for paracetamol, naproxen and bezafibrate (> 1 μg/L), followed by carbamazepine, atenolol, lidocaine, sulfamethoxazole and NACS (<1 μg/L). In hospital wastewater treatment plants (HWWTPs), the highest concentrations measured were for paracetamol and caffeine (> 10 μg/L), followed by ciprofloxacin and NACS (1–6 μg/L), and finally bezafibrate, carbamazepine, atenolol, lidocaine, clarithromycin and sulfamethoxazole (< 1μg/L). Antibiotic drugs were detected in HWWTPs, but rarely detected in MWWTPs. In general, the hospital wastewaters contained relatively higher levels of pharmaceuticals than municipal wastewaters. The removal efficiencies of the pharmaceutical compounds ranged widely. This was found to be related to characteristics and operational parameters of the individual WWTPs. The MWWTPs that utilized long aeration and biomass retention times (HRT,SRT), as evidenced by the occurrence of complete nitrification, were more efficient at removing paracetamol, naproxen, bezafibrate and atenolol, than the non-nitrifying plants with relatively shorter HRT and SRT. HWWTPs that operated under elevated ambient temperatures (> 26°C) achieved higher removal efficiencies (90%) for several compounds, including paracetamol, caffeine, sulfamethoxazole, ciprofloxacin, clarithromycin, NACS, atenolol, carbamazepine and lidocaine. In addition to the elevated ambient temperatures, elevated HRT and SRT and less dilution can lead to increased active biomass and can result in higher removal rates for the pharmaceutical compounds. Overall, the removal efficiencies of pharmaceuticals in WWTPs have been correlated to the type of treatment plant, the plants’ operational parameters (HRT, SRT), the climatic conditions (temperature and dilution effect of rainfall) and characteristics of the micropollutants (type and concentration). Aerobic and anaerobic batch biodegradation experiments were conducted to observe the removal of paracetamol, naproxen, ibuprofen and sulfamethoxazole at various SRTs. The biodegradation rates varied widely ranging from poor, to moderate, to high, depending on the SRT. Paracetamol was highly biodegradable under both aerobic and anaerobic conditions. Sulfamethoxazole was poorly biodegradable under aerobic conditions but highly biodegradable under anaerobic conditions. Relatively slow biodegradation rates were observed for ibuprofen and naproxen under both conditions; longer microbial adaptation periods for these two compounds were probably required. The most important factor affecting the removal of the compounds was the SRT. Therefore, the conclusion was drawn that combining anaerobic and aerobic systems with longer SRT and HRT could bring about significant reductions in the emissions of these contaminants into the environment via WWTPs; this is also a cost-effective option.

This study evaluated the potential for efficient treatment of domestic wastewater, while satisfying energy efficiency requirements. Various treatment systems and the influences of their physical configurations and operational characteristics on wastewater treatment and energy efficiency were initially considered and evaluated. Review of literature identified high rate anaerobic systems as viable low energy systems for domestic wastewater treatment, with reported high removal of influent chemical oxygen demand (COD) and high net energy balance for the anaerobic baffled reactor (ABR). Low energy recovery is reported in literature as a limitation of anaerobic domestic wastewater treatment, and anaerobic domestic wastewater treatment systems have failed to meet effluent discharge standards, and post-treatment using aerobic processes have been recommended in order to ensure high effluent quality. Therefore, the ABR was selected as a feasible option that can be developed as the first stage of an anaerobic-aerobic low energy domestic wastewater treatment system. The literature review also identified the net energy consumption per cubic metre (m3) of treated wastewater during the treatment process as an energy efficiency evaluation criterion. Energy efficiency for domestic wastewater treatment facilities should be achieved if efficient treatment performance can be sustained at ambient temperature, instead of the fixed mesophilic temperature that is commonly adopted in anaerobic treatment processes. To identify an energy efficient design of the ABR in terms of hydraulic retention time and operational temperature, the performance efficiencies of ABR bench models were monitored at ambient temperature and 37oC at hydraulic retention times (HRT) of 48, 36, 24, 12 and 6 hours, which corresponded to organic loading rates (OLR) of 1.25, 1.67, 2.5, 5.0 and 10.0 kg COD/m3 day. 88.43, 90.00, 84.03, 77.01 and 59.35% of the influent COD (mean = 2479.50 mg/L) were removed at 48, 36, 24, 12 and 6 hour HRTs, respectively, in the 37oC bench reactor, while 70.16, 70.36 and 74.99% of the influent COD were removed at 48, 36 and 24 hour HRTs, respectively, in the ambient temperature bench reactor. Steady state performance, in the form of stable pH values, was not observed in the ambient temperature reactor at 12 hours HRT before the end of the bench experiments. Retention of influent total solids was observed to correlate to hydraulic retention time, with increase retention of total solids corresponding to increase in hydraulic retention time. Furthermore, observed total solids retention in the ambient temperature reactor were less than the total solids retention in the 37oC reactor. Anaerobic reduction of domestic wastewater sludge and the corresponding methane production were also evaluated using bio-chemical methane potential (BMP) batch assays at ambient temperature and compared to a fixed mesophilic temperature of 37oC. Low reduction of volatile solids was observed in the BMP assays, with 40% at ambient temperature compared to 56% at 37oC for primary sludge, and 22% at ambient temperature compared to 38% at 37oC for secondary sludge. Critical limitations of the anaerobic stage at ambient temperature were determined to be the biological reduction and conversion of the organic contaminants to soluble COD and volatile fatty acids (VFA). Also, achieving and maintaining steady state performance required a longer time period at ambient temperature than at 37oC, potentially due to the slow growth of the anaerobic microorganisms at ambient temperature. These limitations indicate the need for long (≥ 24 hours) retention periods for efficient operation at ambient temperature. The ABR bench models were evaluated for energy efficiency with the identified energy efficiency criteria, and the operational condition with the highest energy efficiency was determined to be 12 hours HRT at 37oC. Finally, design criteria for the anaerobic stage of the anaerobic-aerobic system were proproposed, along with a process model as a preliminary step for future process research.

Effective management of municipal solid wastes continues to be a major challenge in rapidly growing cities of developing countries. Inadequate waste disposal remains rampant thus posing a serious threat to the environment and public health. The use of biogas technology in the management of municipal solid wastes has become a major focus of interest in developed countries due to its economic value in the generation of energy from biodegradable waste and its ability to divert waste from landfill. The application of the technology in developing countries, in particular sub-Saharan African countries has been a challenge. A major barrier to its implementation in these countries has been attributed to the lack of substantial specific information required in the design of appropriate strategies. This study investigated key factors affecting the application of biogas technology in Kigali City (the capital of Rwanda) with the aim of proffering suitable strategies for effective implementation. The study used an integrated methodological approach interrelating social, technical and economic spheres tools. The study was carried out in three stages. The first stage investigated the availability of suitable feedstock for Anaerobic Digestion (AD) from both domestic and non-domestic sources, household waste management practices and public awareness and attitude towards waste separation at source. Results showed that household food waste was the largest category of waste produced in Kigali City from September 2010 to March 2011, comprising about 74% of total municipal waste produced during this period. This was taken as the average for the City. Socio-economic levels existing in Kigali were found to impact on total waste production with higher production rates occurring in high income households. It was found that households with higher income are more likely to separate waste at source and more willing to pay for waste management services. Incentives, such as free garbage bags, reduced waste charges and economic benefits, with greater focus on poor income communities were found capable of promoting waste separation practice which could effectively enhance the recovery of suitable feedstock. The second stage examined the digestibility and potential energy production from available household food waste and industrial biodegradable feedstocks such as banana and passion fruit wastes. The study also studied the potential effect of operating an AD system under the ambient temperature in Kigali City. Results obtained showed that Kigali ambient temperature of 25oC achieved half methane yields in comparison with the optimum mesophilic operational temperature of 37oC commonly used for large scale AD systems around the world. Results also showed that co-digestion of domestic waste with industrial feedstock can substantially increase biogas yields. The third stage investigated technical requirements and economic viability of operating a biogas plant at 37oC with the available feedstock for the generation of electricity to export to grid and digestate for spreading on land. The annual net energy expected to be generated using the available feedstock was estimated to be 182 kWh/tonne of source-separated waste. Risk analysis revealed that socio-economic needs of communities need to be appropriately integrated to secure the reliability of suitable feedstock in order to ensure project’s viability and profitability. It was also found that the development of biogas technology in Kigali could benefit both technically and economically from current Rwandan National key policy objectives related to municipal waste management. All the above mentioned outcome of the research was used to develop a strategic policy-based operational and investment framework for the application of biogas technology in Rwanda.