Ensuring water re-use projects succeed — institutional and technical issues for treated wastewater re-use

The increasing need for water in the arid areas of the world has resulted in the emergence of new water re-use technologies. The success of water re-use projects however, does not just depend on the effectiveness and suitability of the technology, but also on the presence of an institutional framework that ensures that the treated water can be distributed and used safely and efficiently. These two diverse issues were examined by MWH in projects carried out in Egypt and in the USA. One project in the East Bank of Cairo assessed the potential benefits of treated wastewater for irrigation in agricultural schemes. The project established a lack of clarity on ownership and management of the treated wastewater and outlined the institutional development and legislation necessary for the safe and controlled re-use of treated wastewater. The project demonstrated enhanced crop yields but that additional treatment of the wastewater was required to improve its microbiological quality to the standards required by both international and Egyptian legislation for safe re-use. Another project carried out in California, USA assessed the new technology of submerged membrane bioreactors (MBRs) for water re-use. MBRs combine activated sludge treatment with a membrane separation process. The project studied two commercially available submerged MBR systems at pilot scale. The project was designed to evaluate the feasibility of using permeate from the MBRs treating municipal wastewater as a feed source for thin film composite reverse osmosis (RO) membranes. The MBRs were examined in both nitrification and denitrification mode and both MBR systems showed good removal of BOD, TOC and microorganisms. They both produced a high quality effluent that could be used by the RO membranes for water re-use.

It can be seen that the implementation and monitoring of treated wastewater projects is potentially complex and fraught with institutional ambiguities. The national coordinating committee mentioned above should be the main forum for developing, establishing and changing procedures governing treated wastewater and sewage sludge re-use projects throughout Egypt. The organizational structure shown in Fig. 1 relates specifically to Cairo but can equally well be adapted for other regions. 13. Treated wastewater quality standards The wastewater quality achievable in practice depends on the treatment processes provided at any particular treatment plant and it is essential to match the use of the final water requirements with that level of quality. From the point of view of wastewater re-use in agriculture, however, additional quality characteristics important for health and agronomic reasons are necessary including bacteria, viruses, helminthes, protozoa and physico/chemical parameters such as conductivity and the sodium absorption ratio. Primary treatment of municipal wastewater will remove primarily settleable solids together with any adsorbed or entrained materials, such as heavy metals, which might be associated with the solids. The effect of primary treatment on health and agronomic parameters is of minor significance, except that there may be a high level of toxic heavy metals accumulated in the sludge. Conventional secondary treatment of sewage in biological filters or activated sludge plants is designed to remove more of the biologically degradable organic material, and typically removes up to 80–90% of the BOD5 remaining after primary treatment. Again, the health and agronomic parameters are little affected by conventional secondary treatment processes. Further upgrading of secondary effluent is possible in tertiary treatment processes but complex combinations of unit processes are required to achieve a high quality of effluent for unrestricted use in agriculture. Stabilization ponds can achieve high quality effluent standards with low cost, easily operated systems but the land take is high. In order to meet the need for highly quality treated wastewater new technologies are being developed and studied throughout the world. 14. New wastewater treatment re-use technologies The increased need for re-used or reclaimed water in arid environments has resulted in the emergence of new wastewater re-use technologies. The membrane bioreactor (MBR) is one of these new technologies that combine activated sludge treatment with a membrane separation process. The reactor is operated in a similar way to conventional activated sludge (CAS) but a clarifier is not needed. Instead, a low-pressure membrane, either a micro filter (MF) or ultra filter (UF), is used to perform the sludge separation. The combination of an activated sludge and membrane process produces water that has undergone secondary, tertiary and low-pressure membrane treatment using only one unit operation. An MBR is shown schematically in Fig. 2 and is shown alongside a conventional wastewater treatment plant. The MBR process has shown to provide high quality effluent with high BOD removal, complete nitrification and partial denitrification (Kishino et al., 1996; Fan et al., 1996; Cicek et al, 1998). The MBR process has also shown good TOC removal, and complete TSS reduction (Cicek et al, 1998). The MBR effluent also has the added advantage of being low in turbidity and SDI values, making it possible to use as feed water to an RO system. The submerged MBR configuration has a lowpressure membrane submerged in the reactor and operates under vacuum pressure. The membrane is agitated by coarse air that assists in preventing solids build up at the membrane surface. The submerged membranes are also systematically backwashed, and are chemically cleaned when operating pressures become too high. There are currently three companies that are marketing the submerged MBR configuration: Mitsubishi Rayon Corporation (Mitsubishi) from Japan, Zenon En- vironmental Systems, Inc. (Zenon) from Canada, and Kubota Corporation (Kubota) from Japan. The MBR process has been implemented as full-scale installations all over the world [4]. To date, the largest MBR installation operating on municipal wastewater in the United States is a Zenon system in Arapahoe County, Colorado. 15. Evaluation of the feasibility of the MBR for water re-use The City of San Diego is currently evaluating various water reclamation projects at their Aqua 2000 Research Center. Membrane filters typically cost more than secondary clarifiers of comparable hydraulic capacity. As a result, up until the present time, MBRs have been most successful in the treatment of concentrated wastes where the biological reactor is large but the hydraulic capacity (i.e., the size of the membrane system) is small. As membrane technology is being more developed and reclamation projects requiring desalting membranes, the costs of MBRs are being reduced. That is because studies at both Aqua 2000 Research Center and the Water Factory 21 in California have confirmed that membrane filtration is the most cost-effective and reliable treatment process Tertiary treatment UF or MF Conventional Wastewater Treatment Plant Screen Pretreatment Muncipal Wastewater Sludge Membrane Bioreactor (MBR) UF or MF Fig. 2. Comparison of an MBR with a conventional treatment process.