Municipal sludge digester for co-digestion of primary sludge and high fat industrial waste

CPG (NZ) Ltd assist the Palmerston North City Council (PNCC) with the upgrade of its two municipal sludge digesters at the Totara Road WWTP. The upgrade is from municipal sludge digestion to co-digestion of municipal biosolids and selected agro-industrial co-substrate waste such as cheese whey, piggery manure and dairy wastewater treatment flotation foams. This creates added revenue from collected gate fees and sales of renewable electricity. A staged program for the digester system upgrade was proposed by CPG in 2008 that allowed the smooth upgrade in both digester tanks without interruption of the WWTP operation. Stage I of the upgrade added a low cost temporary improved biogas mixing system to the operating digester 1 giving PNCC the capacity to treat the full primary sludge load of Palmerston North in a single digester tank. Stage II added a new permanent hydraulic mixing system to digester 2 while digester 1 treated the full primary sludge load of Palmerston North at 15 days hydraulic residence time (Thiele 2009). In stage III of the upgrade, Digester 1 receives its own hydraulic mixing system while Digester 2 treats the full primary sludge load of Palmerston North. At this point the mixing system upgrade is expected to double the previous digester gas output to over 90 m 3 methane/hour. In stage IV of the upgrade, a recuperative sludge thickening module (booster technology) will be added to the codigestion system boosting the mixed liquor sludge solids levels in both digester tanks from 1.5 % to about 3 % total suspended solids (TSS). After completion of the upgrade the daily biogas production capacity is improved through improved mixing and the booster technology from 45 m3 methane/hour (primary sludge only) to about 180 m3 methane/hour. The additional biogas is produced through co-digestion of primary sludge with up to 100 t/day of additional co-substrate waste.

This paper is the follow-up/validation of the concept paper presented in the 2009 Water NZ conference on the digester upgrade and indicative costs (Thiele, 2009). We present the results from the digester operation with primary sludge before and after the mixing system upgrade. The new hydraulically mixed digester tank (stage II) produced better digester contact than the biogas recirculation mixed tank and achieved comparably low volatile fatty acid (VFA) levels (less than 30 mg VFA/liter), a higher pH and higher mixed liquor alkalinity. This indicated improved primary sludge stabilization with the new hydraulic mixing system. Selective addition of high fat content dairy wastewater flotation foams to stage II practically doubled the biogas production, while the volatile fatty acid levels remained low (less than 40 mg VFA/liter) consistent with improved sludge:biomass contact after the digester upgrade with the hydraulic mixing system.

A mass balanced sludge digester process computer model is then used to determine the additional gas production that can be achieved with addition of recuperative sludge thickening (“digester booster technology”) to the upgraded digesters. Depending on the nature of the industrial waste mixture, a biogas production improvement from the current base of about 1,700 m3 biogas/day (no co-digestion) to 5,400 - 7,900 m3 /d was estimated by the process model under a wide range of different booster operating conditions. The computer model results and a range of suitable co-substrate mixtures for the booster technology are presented. The results are now used for the implementation of digester booster technology at the PNCC municipal sludge digesters.