A trial showing that digestion with recuperative thickening is effective to reduce digester process inhibition risks and improve FOG loading rate in municipal digesters.
JH Thiele, P Burt, M Monaghan
Publication Date (Web): 13 July 2016

The combined anaerobic digestion (co-digestion) of municipal wastewater treatment plant biosolids and trade waste (food residuals, selected industrial processing waste and grease trap waste) to biogas and dewatered sludge allows the provision of up to 100% of the heat and power requirements at municipal treatment plants.
Co-digestion of trade waste materials with high fat, oil and grease (FOG) content often results in lower waste procurement/transport costs, a higher methane yield, improved biogas quality and improved commercial viability of the co-digestion plant. However, addition of FOG trade waste increases risks for digester souring, digester foaming and fat deposition in tanks and pipework. Digester operation guidelines for FOG co-digestion therefore recommend low FOG loading rates (< 0.5 kg FOG/m3digester/day).

Recently we showed that integration of recuperative thickening (RT) doubled the biosolids treatment capacities in municipal sludge digesters. Here we show that RT during FOG waste co-digestion removes digester process inhibition risks, increases the digester process stability and maximises digester gas production. With the use of RT in municipal sludge digesters, FOG digestion capacities were improved from 0.5kg FOG/m3digester/day to > 1.5kg FOG/m3digester/day and biogas productivities increased from 0.7m3biogas/m3digester/day to > 2.3m3biogas/m3digester/day. Between 80% and 95% of the added FOG trade waste solids were converted to biogas. The gas production increase responded to the FOG waste addition within four to six hours. The volatile fatty acid (VFA) levels in the FOG trade waste digesters with recuperative thickening remained low (20–30mg VFA/L), confirming the robustness of the improved FOG digester process.

This process was operated at the treatment plant digesters in Palmerston North, New Zealand, for over three years without digester process issues, foaming issues or mechanical problems. The co-digestion biogas was used for electricity production in co-generation. Typically, more than 100% of the treatment plant power requirements were provided by the co-generation. Operation data showed that the trade waste reception of the test plant worked consistently well when receiving dairy factory waste with high FOG content (70% FOG in dry matter).

In conclusion, the operating experience from the sludge digesters at the Palmerston North treatment plant has shown that digester upgrades with recuperative thickening improve the technical, economic and environmental sustainability of trade waste co-digestion programs in municipal digesters. The RT-based digester process allows an increase in FOG loading rates and the average daily biogas output from municipal digesters by a large margin (> 200 % increase). Sludge digester productivity and gate fee income for treated trade waste were typically three-fold improved. Typical payback periods for the added trade waste reception and recuperative thickening plant in this improved trade waste digestion process was less than four years.


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