A case study for a cost-benefit-based, stepwise optimization of thermo-chemical WAS pre-treatment for anaerobic digestion

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 methanepotentials 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.

A Review of Pretreatment Methods to Enhance Solids Reduction during Anaerobic Digestion of Municipal Wastewater Sludges and the Resulting Digester Performance: Implications to Future Urban Biorefineries

“…A study by Nagler et al on the pretreatment of WAS over a temperature range between 39 and 200 • C and alkaline treatment dosing NaOH over different concentrations reported an optimal treatment condition for WAS pretreatment of 70 • C and 0.04 M NaOH mixed for one hour. This pretreatment strategy reduced sludge production post-AD by increasing the sCOD by 34% when compared to the untreated WAS but only increased the sludge degradation by 2% when compared to thermally treated sludge at 70 • C. This reduction resulted in the lowering of annual biosolids disposal costs by 21% [61]. Yi et al using WAS achieved 26% reduction for VSS and 200% for sCOD by using alkali dosing of 0.05 g NaOH/g TS that was mixed/reacted for 24 h followed by thermal treatment step using a temperature of 70 • C that was then reacted for either 2 h and 9 h under mixed conditions [34].…”

“…The integration of thermal methods with chemical addition reduces the consumption of chemicals by up to six times over chemical addition alone, while achieving improved solubilization at the higher temperatures compared to chemical pretreatment alone. The use of thermal methods in combination with an alkali has been extensively studied as an integrated pretreatment system [34,40,61]. Acid has also been used in conjunction with thermal treatment; however, integrated alkaline/thermal pretreatment systems have shown the maximum VSS reductions.…”

Design of biogas digester with thermophilic pretreatment for reducing fruits wastes

“…Pre-treatment (PT) of active waste sludge (WAS) was optimized by thermochemical treatment at 70 °C and 0.04 M NaOH solution for 1 h. The maximum PT efficiency in sludge disintegration can be achieved in combination with heat treatment and low chemical-related costs. Some advantages of the PT were increasing the yield of methane and biogas, causing increased degradation of sludge during anaerobic digestion, the drainage capacity of higher digestible sludge, and reducing the amount of waste to be disposed of in relation to WAS from waste water treatment plants (Nagler et al 2018).…”

Enhancement methane generation by co-digestion of cow dung and grass (Cynodon dactylon) with activated sludge

“…Thus, the best route to improve biogas production is the anaerobic co-digestion (AD) process (Hassan et al 2016b). Thermochemical pre-treatment for different crops is beneficial for the improvement of methane because treating the biomass can increase the hydrolysis stage, which will result in a greater production of methane (Bhatnagar et al 2018;Nagler et al 2018). Corn cob pretreated with alkali and enzymatic hydrolysis showed a 23% increment in methane yield compared with raw corn cob (Pérez-Rodríguez et al 2017).…”