A systemic approach to implementation of sanitation and agricultural water reuse

Abstract

In the light of a growing global urban population and increasing water scarcity, previous work in the field of sanitation and water reuse has emphasized the need for a holistic and integrated view of all components involved in sanitation systems. A systemic approach is needed to recover the resources contained in sewage and to maximize their benefits. There is a consensus that this approach requires further support. The main obstacles to the implementation of new approaches to urban water management are the lack of knowledge regarding inherent uncertainties and risks, practical management challenges, the available institutional capacities and capacities to facilitate community involvement, financial considerations, and institutional and personal biases that act as barriers. This dissertation explores the main challenges and knowledge gaps encountered during implementation of a project on sanitation and water reuse in North Namibia. The objective is to fill the encountered knowledge gaps and to demonstrate the implications that the observations have in practice. This reduces obstacles to the implementation of new concepts in urban water management and maximizes the achievable benefits of sanitation systems. The implemented infrastructure includes various types of sanitation facilities, a vacuum sewer system, a wastewater treatment plant with sedimentation and anaerobic pretreatment, aerobic treatment and secondary clarification, microscreening and UV disinfection. The treated water is stored in a pond and applied to agricultural fields via surface drip lines. The reclaimed water is used for the production of vegetables for human consumption. The main results of this study and its consequences for practice can be summarized as follows: •The specific water use and the specific loads in wastewater from shared sanitation facilities differ considerably from those of individual sanitation facilities. Hence, the wastewater characteristics are also different, which has implications for wastewater and sludge treatment, nutrient and salt management for water reuse on agricultural fields and the energy recovery potential from the wastewater constituents and agricultural biomass. •The structural layout of shared sanitation facilities needs to fit with the desired management and billing system. Particularly important aspects are the collection of revenues and control of visitor flows. •Tariff levels, the method of revenue collection, and the population density influence the utilization and hence the quantities and characteristics of the wastewater from shared sanitation facilities. This already needs to be considered during planning. Generation of sufficient revenues for cost recovery is difficult in low-income areas with a low population density. •In settings where national regulations regarding reclaimed water quality do not (yet) ex-ist, the recommendations in this study can be used to develop relevant water quality criteria. The recommendations in existing international guidelines are complemented with site-specific water quality limits for the protection of irrigation infrastructure (turbidity, chemical oxygen demand, biochemical oxygen demand), the required water quality prior to UV disinfection (turbidity, total suspended solids, particle size), and prevention of eutrophication and negative effects on plants (nitrogen, phosphorus and potassium). •Water storage facilities should be considered as an additional treatment step that contributes to the reliability of the water reclamation process and to achieving the required water quality. •The risks of soil salinization and overfertilisation were less serious than expected in this case. However, in other settings with, e.g., a higher proportion of wastewater from individual households, measures for control of salts and nutrient input to agricultural fields need to be implemented as suggested. •Residues of crops irrigated with reclaimed water can contribute only to a limited extent to biogas and electricity generation via codigestion with sewage sludge. The market value of the crops is usually higher than the value of the producible electricity. •Important impediments to co-generation in Southern Africa are the tariff structures of the local electricity supply entities. Rebates or credits for electricity fed into the grid are usually not possible. Additionally, fixed costs constitute a major part of the electricity costs. Thus, for the given tariff structure, co-generation can only reduce electricity costs if the produced electricity is consumed immediately on site. For the first time, a sanitation system has been analyzed from a holistic perspective, providing detailed specifications for planning, data monitoring and influencing factors. This is a sound basis for better planning and implementation of similar projects. The knowledge gaps that caused misconceptions and difficulties during realization of this project are now closed or addressed and can, at least, be realistically assessed right from the start.