Water Journal : Water Journal May 2011
biosolids refereed paper 90 MAY 2011 water technical features Abstract Changes in wastewater treatment design, with longer sludge ages, mean that waste solids streams are becoming more difficult to stabilise by conventional anaerobic processes. In this article, we focus on a variety of pre-treatment methods that can improve anaerobic digestion performance, either by increased speed of degradation, or by an increase in ultimate degradability. In particular, we focus on biological pre-treatment (acid-phase and temperature), which mainly improves the speed of degradation. This can result in a substantial increase in gas production and an increase in VS destruction from 30%--45% depending on the main digester retention time. We also show that it is important to assess how pre-treatment affects the final solids, as this can determine the ultimate performance. Biosolids Source, Production Levels and Reuse The major solids streams produced by wastewater treatment plants are primary sludge (separated by the primary clarifiers), and excess waste activated sludge (WAS) (separated in the secondary clarifiers). The average production of dry solids is 50--65 grams per person, per day (Hudson 1995). A community of 50,000 persons will, therefore, produce 2.5 dry tonnes of biosolids per day, or approximately 20 wet tonnes per day (one truckload). This depends heavily on biosolids handling methods and wastewater process design, rather than upstream consumer habits (in contrast with total sewer flows). Biosolids handling is expensive and represents 25%--50% of total wastewater treatment plant costs (Murthy, Higgins et al., 2006). Costs are heavily related to final handling costs (mainly transport), and are (as of 2011 in Australia) in the order of $35--$70 per wet tonne depending on the distance for final application. This is complicated in Australia by the fact that our activated sludges are inherently difficult to dewater, possibly related to inert solids and/or a higher bound water fraction (Novak, 2006). In the last 15 years, the focus of wastewater treatment has shifted more towards nitrogen removal, with a concurrent increase in sludge ages. This has had the following major impacts: • Removal of the primary sedimentation processes, or use of sludge pre- fermenters, as the carbon is needed for nutrient removal. • Decrease in overall biosolids amounts, due to long sludge ages and loss of the primary stream, though the secondary sludge production normally increases due to increased carbon turnover. DJ Batstone, PD Jensen, H Ge Pre-treatment methods such as biological processes can improve performance economically BIOCHEMICAL TREATMENT OF BIOSOLIDS -- EMERGING TECHNOLOGIES Table 1: Treatment options available in different wastewater treatment plant sizes. Type Stability Class Product Quality Electrical Use (kWh/tonne at 13% dry solids) Notes Treatment -- applicable at all sizes Lime stabilisation AorB4 ++ 400--8001 Increases dry solids 50%. Causes high pH solids. Composting A +++ 100 High labour requirements. Increases dry solids 50%. Needs additional dry material. Aerobic digestion B -/+ 50 25 days Treatment -- applicable at larger scale Anaerobic digestion 30°C 40°C B + -35 Sludge age ~15 days. Co-generation gas available. Anaerobic digestion with pre-treatment A/B ++ -25 to -50 Post-treatment -- applicable at all sizes Solar drying2 A/B +++ 30 Based on continuous turning process. Thermal drying2 A +++ 10003 1 Including electrical cost of lime production. Upper level (and Class A) includes heat treatment. 2 kWh per tonne water evaporated. 3 If gas is used, emissions are equivalent to 200kWh as electricity. 4 Class A (P1) is pathogen-free. Class B (P2) is stabilised biologically but may contain residual pathogens.
Water Journal April 2011
Water Journal July 2011