A case study for a costbenefitbased stepwise op

Authors: Magdalena Nagler Peter Aichinger Martin Kuprian Thomas Pümpel Heribert Insam Christian Ebner

Publish Date: 2016/12/21

Volume: 20, Issue: 1, Pages: 266-273

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Abstract

Wasteactivated sludge WAS may be considered a resource generated by wastewater treatment plants and used for biogasgeneration but it requires pretreatment PT for enhanced biogasyields 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 thermochemical PTstrategy in terms of net energy output and costefficiency by optimizing PT temperature and the amount and sort of the alkaline reagent used We compared methanepotentials and disposal costs of untreated and treated WAS and conducted an annual costbenefit calculation We defined 70 °C and 004 M NaOH as ideal PTconditions being both lowenergy demanding and efficient Applying these conditions enhanced biogasyields and improved dewaterability led to reduced electricity and disposal costs of 22 and 27 respectively resulting in savings of approx 28 of the yearly WASrelated expenditures of a wastewater treatment plant Despite multiple benefits in running costs the implementation of WASPT was not recommendable in the presented case study due to high investment costsWasteactivated sludge WAS is formed at a consistent rate during the treatment of wastewater representing around 30 w/w of the chemical oxygen demand CODload of waste water treatment plants WWTP 1 To minimise odor volume disposal costs and biological activity of the sludge and to harvest energy WAS is often used at WWTP to produce biogas This methanerich gas is used to supply heat and electricity for the wastewater treatment process and buildings Although the biodegradability of WAS by anaerobic digestion varies with certain characteristics such as CODTOC ratio carbohydrate protein and lipid concentrations the biogas yield is generally relatively low with 50–200 LCH4 kg volatile solidsVS −1 compared to 600–800 LCH4 kg VS −1 for municipal organic waste 2 3 4 Pretreatment of this substrate however shows great advantages and may allow an immediate and distinct increase in methane yields 3Due to these low native energy yields but promising increases after PT WAS is in fact one of the widest studied substrates in terms of pretreatment PT strategies such as biological thermal microwave mechanical ultrasonic pulsed electric fields freeze/thaw chemical and wet oxidation 5 Of all these strategies thermal and chemical PT seem to be the most efficient in terms of fullscale applicability efficiency and economic profit 6 7 8 Up to now several studies have been conducted focusing on either single or combined PT using heat and alkaline reagents Regarding the temperatures ranges from 25 to 275 °C have been investigated and for the alkaline reagent several chemicals KOH CaOH2 NaOH in different concentrations were tested 5 9

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