Abstract
Waste-activated sludge (WAS) may be considered a resource generated by wastewater treatment plants and used for biogas-generation but it requires pre-treatment (PT) for enhanced biogas-yields and reduced WAS disposal costs. To date, a number of studies on the optimization of such PT focused on improved biogas yields but neglected inferred energy and resource consumption. Here, we aimed to identify the most promising thermo-chemical PT-strategy in terms of net energy output and cost-efficiency by optimizing PT temperature and the amount and sort of the alkaline reagent used. We compared methane-potentials and disposal costs of untreated and treated WAS and conducted an annual cost-benefit calculation. We defined 70 °C and 0.04 M NaOH as ideal PT-conditions being both, low-energy demanding and efficient. Applying these conditions, enhanced biogas-yields and improved dewaterability led to reduced electricity and disposal costs of 22 and 27%, respectively, resulting in savings of approx. 28% of the yearly WAS-related expenditures of a wastewater treatment plant. Despite multiple benefits in running costs, the implementation of WAS-PT was not recommendable in the presented case study due to high investment costs.
Highlights
Waste-activated sludge (WAS) is formed at a consistent rate during the treatment of wastewater, representing around 30% (w/w) of the chemical oxygen demand (COD)load of waste water treatment plants (WWTP) [1]
(2) The suitability of biomass ash, aluminate and NaOH as alkaline reagents in various concentrations was tested and the treatment yielding highest degrees of disintegration in combination with low energy input was defined as optimum PT. (3) For this most promising PT, the resulting WAS biological methane potential and dewaterabilities were determined in order to (4) conduct a cost-benefit calculation including all WAS-related costs for pre-treated and native WAS
The samples were taken after polymer dosage and mechanical excess sludge dewatering with a final content of total solids (TS) from 4 to 6% (w/w), volatile solids (VS) of 70% and a pH of 7
Summary
Waste-activated sludge (WAS) is formed at a consistent rate during the treatment of wastewater, representing around 30% (w/w) of the chemical oxygen demand (COD)load of waste water treatment plants (WWTP) [1]. Volume, disposal costs and biological activity of the sludge and to harvest energy, WAS is often used at WWTP to produce biogas. This methane-rich gas is used to supply heat and electricity for the wastewater treatment process and buildings. Pre-treatment of this substrate, shows great advantages and may allow an immediate and distinct increase in methane yields [3]. Due to these low native energy yields but promising increases after PT, WAS is one of the widest studied
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