Abstract
The first step of anaerobic digestion, the hydrolysis, is regarded as the rate-limiting step in the degradation of complex organic compounds, such as waste-activated sludge (WAS). The aim of lab-scale experiments was to pre-hydrolyze the sludge by means of low intensive alkaline sludge conditioning before applying hydrodynamic disintegration, as the pre-treatment procedure. Application of both processes as a hybrid disintegration sludge technology resulted in a higher organic matter release (soluble chemical oxygen demand (SCOD)) to the liquid sludge phase compared with the effects of processes conducted separately. The total SCOD after alkalization at 9 pH (pH in the range of 8.96–9.10, SCOD = 600 mg O2/L) and after hydrodynamic (SCOD = 1450 mg O2/L) disintegration equaled to 2050 mg/L. However, due to the synergistic effect, the obtained SCOD value amounted to 2800 mg/L, which constitutes an additional chemical oxygen demand (COD) dissolution of about 35 %. Similarly, the synergistic effect after alkalization at 10 pH was also obtained. The applied hybrid pre-hydrolysis technology resulted in a disintegration degree of 28–35 %. The experiments aimed at selection of the most appropriate procedures in terms of optimal sludge digestion results, including high organic matter degradation (removal) and high biogas production. The analyzed soft hybrid technology influenced the effectiveness of mesophilic/thermophilic anaerobic digestion in a positive way and ensured the sludge minimization. The adopted pre-treatment technology (alkalization + hydrodynamic cavitation) resulted in 22–27 % higher biogas production and 13–28 % higher biogas yield. After two stages of anaerobic digestion (mesophilic conditions (MAD) + thermophilic anaerobic digestion (TAD)), the highest total solids (TS) reduction amounted to 45.6 % and was received for the following sample at 7 days MAD + 17 days TAD. About 7 % higher TS reduction was noticed compared with the sample after 9 days MAD + 15 days TAD. Similar results were obtained for volatile solids (VS) reduction after two-stage anaerobic digestion. The highest decrease of VS was obtained when the first stage, the mesophilic digestion which lasted 7 days, was followed by thermophilic digestion for 17 days.
Highlights
Anaerobic digestion (AD) of waste-activated sludge (WAS) is commonly used in the wastewater sludge treatment, and it is an appropriate technique for the treatment of WAS, before final disposal
The WAS disintegration was performed in two steps, chemical lysis of microorganism cells, followed by hydrodynamic disintegration, called hybrid disintegration process in the latter part of the paper
This is in contrast to many other investigations where the increased of pH was continually maintained for the entire period of digestion
Summary
Anaerobic digestion (AD) of waste-activated sludge (WAS) is commonly used in the wastewater sludge treatment, and it is an appropriate technique for the treatment of WAS, before final disposal. This technology is employed worldwide as the oldest and most important process for sludge stabilization (Dohányos and Zábranská 2001; Rahmani et al 2009). The first step, the hydrolysis, leads to solubilization of insoluble particulate matter and biological decomposition of organic polymers to monomers or dimers It is a rate-limiting step of AD, when complex organic material is degraded and most of the biodegradable material is either enclosed in the microbial cell wall or enmeshed in an extracellular polymeric matrix. This contributes to limiting biodegradability of these WAS and limits the Environ Sci Pollut Res (2015) 22:7258–7270 volatile solids (VS) reduction to the level of 35–45 % (Bolzonella et al 2005; Bhattacharya et al 1996; Coelho et al 2011)
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