Using a membrane bioreactor/reverse osmosis system for indirect potable reuse

The City of McAllen, Texas, with the assistance of CH2M HILL, has pilot tested an integrated membrane bioreactor (MBR)/reverse osmosis (RO) treatment train to reclaim municipal wastewater to a quality suitable for use as a new drinking water supply in the process called indirect potable reuse. Previous testing by the City (Phase 1) demonstrated the applicability and cost of microfiltration (Memcor and ZeeWeed systems) to enhance the quality of secondary effluent for subsequent treatment by RO and the feasibility of a membrane bioreactor system (ZenoGem) to produce RO feedwater directly from minimally processed sewage. Phase 2 testing, reported on in this paper, is designed to demonstrate reliable operation of MBR/RO treatment for processing screened, degritted sewage and that the effluent from such a train can meet all federal primary and State secondary drinking water regulations and comply with anticipated State requirements for indirect potable reuse. Results show the ZenoGem process to be reliable, require minimal operator attention and maintenance, produce an effluent that can be processed by RO with little fouling and that easily exceeds the City's current effluent discharge requirements relative to BOD, TSS and ammonia. The ZenoGem permeate quality exceeds RO feedwater criteria for turbidity and silt density index and RO system performance confirms minimal membrane fouling by particles. However, the high calcium hardness and phosphate levels in the City's wastewater (and ZenoGem permeate) caused mineral precipitation within the RO system when operated at higher recoveries. Precipitation can be controlled, however, by increased acidification of the RO feedwater.