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Wastewater Treatment
Aerial photograph of the Lions Gate WWTP showing Digester 4 in the foreground.
feeding with thickened primary sludge (TPS) began. Throughout the start-up period the average total solids (TS) percentage of the feed sludge was 5.9%, with volatile solids (VS) of 91%. Various parameters were monitored by the operations team on a daily basis, including pH, volatile acids (VA), alkalinity, temperature, flow rate, TS% and VS% of the digester feed and digested sludge, gas production, hydraulic load (days) and organic loading (kg/m3.d), with the volatile fatty acids-to-alkalinity ratio (VA:Alk) and pH key indicator parameters used to assess digester stability. Stable operation was recorded with pH value between 7.3 and 7.9, and a VA:Alk ratio between 0.1 and 0.16 with an average of 0.12. Despite some fluctuations early on in digester temperature, the VA:Alk ratio never went above 0.2 and the process remained stable. After a period of 35 days, all of the TPS produced on-site was being pumped to Digester 4. After 61 days, stable operation of Digester 4 had been confirmed, so that Digester 3 could be isolated and emptied for scheduled maintenance. Conclusion The newly upgraded Digester 4 is currently receiving, on average, 167 m3/d of TPS, and is producing 5,412 m3/d of biogas. This will allow Digester 3 to be taken out of service for maintenance and refurbishment. Digester mixing and transfer systems are now fully automated in the WWTP’s computerized data acquisition and control system, following internal Metro Vancouver programming. Based on long-term operational experience, thermophilic anaerobic digestion has proven to be a sustainable and robust system, offering a reliable and cost-effective method for sludge treatment and energy recovery at the Lions Gate Plant. The continued use of this renewable energy source is one of the many Metro Vancouver Sustainable Region Initiatives, that promote energy conservation and reductions in greenhouse gas emissions. Caroline O’Reilly, Leif Marmolejo, Brandon Walker and Christian Brumpton are with Associated Engineering. For more information, E-mail: oreillyc@ae.ca
44 | September 2011
Environmental Science & Engineering Magazine