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Sustainable Wastewater Treatment: Optimizing Options and Outlook of Opportunities

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Shizas and Bagley (2004) measured the potential energy in wastewater to be up to nine times greater than the electricity needed to operate a municipal WWTP. In contrast, conventional wastewater treatment plants have difficulty to generate enough energy to cover about half of their needs, although recent efforts have allowed to raise the typical energy recovery from 49% to energy autonomy in some conventional plants: Wett and Dengg (2006) show an electricity production of 108% of the energy needs in a plant in Strass, Austria. This was achieved by a combination of methods, driven through a country-wide benchmarking exercise. Optimising the liquid treatment, the specific energy consumption per population equivalent (60 mg BOD /PE/ is as low as 23 kWh/yr, which could be counterbalanced by optimum digestor biogas conversion.

Another way to harvest the energy potential of wastewater is to convert it directly into methane through anaerobic pre-treatment, and to generate electrical energy by converting both incoming soluble organics and solids directly to biogas. If sewage is directly treated anaerobically, the typical conversion at favourable temperatures >20 deg C is 0.35 m3 of CH4/kg of COD removed (Chernicharo, 1997). Therefore every m3 of wastewater, based on a raw COD of 600 mg/l, and a removal of 65%, will yield a methane volume of 0.14 m3.

With conventional primary and secondary treatment, the residual solids from both steps will provide the energy source. Based on typical values in Europe and the US, the volatile solids (VS) of the primary sludge are between 70 to 80% and activated sludge has 75 to 85% VS. Thus, when the solids from primary and secondary treatment are converted in anaerobic solids digesters, the typical gas production for a BOD of 300 mg/l would be:

primary settler efficiency of 50% solids yields 150 mgTSS/l at 75% VS

the biological reactor would be fed with 200 mg BOD/l

the biomass yield would be 0.7 mg TSS/mg BOD × 80% VS

destruction of volatile solids in the digester at 50%

Biogas yield per solids destroyed at 1 m3 / kg VS destroyed

Total biogas from primary and bio-solids is 0.11 m3 per m3 of influent

Methane content is 65%, specific CH4 production is 0.07 m3 / m3

This comparison shows that the direct anaerobic digestion of sewage yields up to twice as much biogas as oxidising pollutants into solids, then extracting and digesting them. In addition, as twice as much BOD is removed in the first anaerobic treatment step, only half of the energy is needed to oxidize the remaining 105 mg BOD/l. As shown in Table 1, anaerobic treatment followed by aerobic polishing is therefore a large net energy producer.

Upflow Anaerobic Sludge Blanket (UASB) reactors were first developed in the 1970s for the treatment of highly concentrated industrial wastewater, such as sugar and brewery waste. The benefits of the system, such as low sludge production, small footprint, low energy requirements and valuable biogas production, made the UASB reactors attractive for highly concentrated industrial wastewater, often with warm temperatures (Foresti, 2002). Anaerobic treatment is the key process for the new WWTP in Ajman, one of the United Arab Emirates, with wastewater temperature expected to be around 25°C. The aim of the Ajman project is to provide a centralized wastewater collection and treatment system in the City of Ajman, to replace the current septic tanks, and deliver the treated effluent for use in irrigation. Following pre-treatment of the influent in the form of screening and grit removal, the UASB reactors are followed by submerged aerated filters (SAF) as second biological treatment process. Deep bed sand filters will be used as polishing units after the biological treatment stages to ensure compliance with the strict effluent consent of 10 mg/l BOD and 10 mg/l TSS. As the effluent is to be used as irrigation water, the final stage in the treatment process stream is chlorine disinfection with a requirement of 200 FC / 100 ml.

To optimise water and solids operations, the Life Cycle Assessment (LCA) can be used as a tool to evaluate the environmental impact of a product, process, or activity by examining the energy and materials used, waste generated, and ways of implementing environmental improvements. LCA is not to be confused with Life Cycle Cost Analysis (LCCA), which is a process for evaluating the total economic cost of an asset by analyzing initial costs and discounted future expenditures such as maintenance, operational, user, and social costs over the life cycle of an asset.
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Document Type: Research Article

Publication date: 2008-01-01

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