Abstract
After full-scale trial ran at Oxford STW, Thames Water acquired 19 Bucher Press for 4 different STW with the purpose of hydrolysed digested sludge dewatering. This paper covers experience from installation to operation of the presses and through sludge characteristic change (Conventional Digested sludge to Hydrolysed Digested sludge) at Oxford STW. The discussions include learnings on design for dewatering hydrolysed digested sludge, operational settings, performance assessment parameters and performance optimisation, polymer consumption, power consumption and required maintenance from 01/10/14 to 01/10/15. During this period the presses were being operated by Tier 1 contractors responsible to deliver the sludge stream upgrade project and Thames Water had no major accountability for performance.
Keywords: Hydrolysed Digested Sludge, High Dry Solids Dewatering, Operational Experience
Introduction
High Dry solids Dewatering and Wastewater Industry In Sludge Treatment from the municipal sewage treatment, Thames Water appears to use dewatering technologies for three main applications: a) pre-THP dewatering, in order to get sludge at ideal % Dry Solids (DS) to be fed into Thermo-hydrolysis Processes (THP) reactors; b) cake imports transport between satellite sites and big sludge treatment centres; and c) in the end of sludge treatment, where sludge cake is transport to its final destination.
The technologies historically used for the mentioned above are belt presses, decanter centrifuges and plate presses. Bucher Presses have shown a high capacity to extract liquid in the food market, being also very competitive in the Wastewater Industry. A few presses were then installed in Germany, Switzerland, Sweden and Norway for digested sludge cake production.
Thames Water has run lab-scale trials at its Research & Development Centre followed by full-scale trials in partnership with Kier at Oxford STW. During one year, sewage sludge from existing sites was transported to Oxford to be tested. Table 1 shows a summary of a few key parameters observed during the one year full-scale trial. Anaerobic Digestion or THP + Anaerobic Digestion were the treatment technologies tested, as required by strategic team.
The Bucher Press consists of a large cylinder with a moving piston and parallel filter elements (socks) inside. These socks are made of normal filter cloth with a flexible support structure inside to allow flow through the cloths, down the tubes to the end of the press (e.g. Fountain et al. 2013). Filtrate is extracted from sludge during (1) Filling-pressing and (2) Pressing only cycles. The last cycle of the batch (run) is the discharge, where sludge is pulled out of the cylinder by the pressing piston. The press is ready to recommence the run. What guarantees High Dewatering Solids is the fact that cake is uniformly dewatered during (1) and (2). Cake from belt or plate presses present a skin in contact with the cloth/belt which is much drier, while the cake behind the skin is wetter because the water was less able to get away. A schematics of the described mechanism of dewatering is observed in Figure 1.
The original sludge treatment plant load capacity of 24 TDS/d was consequently expanded to a load of 67 TDS/d. The maximum guaranteed capacity load includes a 15% headroom capacity from the expected load of 58 TDS/d of a PE of around 670,000 expected for the area in 2021.
In the new installed facility, sludge is converged to the pre-THP Buffer Tank (2 Silos) via 2 streams: a) cake imports facility (up to 30TDS/d), with rewetting with sludge or final effluent, according to operations choice, and b) dewatered on site indigenous (22 TDS/d) plus liquid imports (10TDS/d). After the silos, sludge is diluted to 16% DS, optimal solids content for THP feed. Liquid Sludge (Indigenous and Imports) is screened on site by Strain Presses (Huber). Cake Imports are screened by each satellite site. The average % Primary Sludge into the Silos is from 40 to 60% with 35 to 45 g of Iron (Dry Weight) per kg DS.
After hydrolysis (Veolia BiothelysTM plant, with capacity to treat up to 70 TDS/d of sludge; 3 pairs of reactors), sludge is diluted to 8-10% DS and pumped to Digesters 1, 2, 3 & 4 (recirculation loop feed). The digester hydraulic retention time target is 13.5 days. Digested sludge spilled out of digesters goes to a first buffer tank by gravity. It is by this point transferred at 90m3/h through a 120 meters gallery to 2 buffer tanks located in front of the existing post-digestion dewatering building. Those two tanks feed the post-digestion dewatering feed tank, which feeds the post-digestion dewatering presses (Bucher Presses). Figure 2 shows treatment flowchart.
Filtrates from pre-THP dewatering belt presses and post-digestion dewatering Bucher Presses are converged to a balance tank and treated in a Cyclic Activated Sludge System (CASSTM) Liquor Treatment Plant (LTP). The biogas storage was also increased, aiming better usage of expected increase in biogas production. Although the LTP and the biogas system are part of the sludge stream upgrade project, they aren’t represented in Figure 2.
