Abstract
The highly variable characteristics of waste activated sludge (WAS) hinder the comparison of experimental results on WAS bioconversion between the different studies that use excess sludge from different origin. Sludge grown under laboratory conditions with synthetic wastewater as feed showed high resistance to commonly applied pre-treatment techniques, such as thermal pre-treatment. However, a distinctly higher bioconversion of this sludge was recorded compared to WAS from a full-scale wastewater treatment plant (WWTP). The observed results casted concern on the suitability of the experimental laboratory-based data for practice. The physicochemical and biochemical characteristics of both WAS and lab-grown sludge are dependent on the wastewater characteristics or growth media on which the sludges were grown. The objective of this study was to formulate a growth medium that results into a lab-grown sludge which shows high similarity to the WAS coming from a specific full-scale WWTP in response to a pre-treatment technique. More specifically, in this study we targeted the formation of slowly-biodegradable lab-grown sludge that is similarly responsive to mild thermal pre-treatment with H2O2 addition. By comparing real and synthetic wastewaters, we discussed the various wastewater constituents that may lead to a higher degree of recalcitrance of the produced sludge. We then formulated a growth medium, which was fed to a lab-scale activated sludge reactor and evaluated the nutrient removal capacity, as well as the characteristics of the cultivated sludge before and after pre-treatment. Finally, the growth medium was modified to provoke a change in both the bioconversion and in the response to mild thermal pre-treatment. The growth medium proposed in this study resulted in a slowly-biodegradable sludge (195 ± 3.7 NLCH4/kgVSadded) that after thermal pre-treatment resulted in an increase in methane production of 9 %, which was similar to the WAS coming from the full-scale WWTP. It was concluded that not only the bioconversion but also the response to mild thermal pre-treatment of lab-grown sludge was determined by the composition of the growth media.
Highlights
Waste activated sludge (WAS) is the most abundant by-product of conventional activated sludge (CAS) wastewater treatment plants (WWTP) for municipal sewage: a biological matrix composed of mi crobes, metals and countless unknown organic and inert materials (Tchobanoglous et al, 2003)
At the start of the anaerobic phase, the concentrated synthetic wastewater was fed from the bottom of the sequencing batch reactor (SBR) for 7 min and tap water was added during 60 min to reach a chemical oxygen demand (COD) ≈500 mg/L
The evidence presented in this study suggests that the biodegradation of lab-grown sludge varied according to the character istics of the growth media, and that the effect of pre-treatment depended on the inherent biodegradation of the slowly-biodegradable fraction of the sludge (Fig. 7)
Summary
Waste activated sludge (WAS) is the most abundant by-product of conventional activated sludge (CAS) wastewater treatment plants (WWTP) for municipal sewage: a biological matrix composed of mi crobes, metals and countless unknown organic and inert materials (Tchobanoglous et al, 2003). A systematic comparison between the different WAS pre-treatment tech niques demands the availability of sludge with defined non-variable characteristics with a high similarity in physicochemical and biochem ical properties with WAS from a full-scale CAS system. For this reason, various researchers cultivated lab-grown sludge that, differed substantially in composition and characteristics (Dai et al, 2017b; Xu et al, 2017). Since the studied pre-treatment methods generally increase the WAS bioconversion from full scale WWTPs (Gonzalez et al, 2020; Pilli et al, 2014), it was hy pothesized that the inherently high bioconversion of the lab-grown sludge severely masked the effectiveness of pre-treatment
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