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Achieving membrane plant startup in a remote location
Canaan Valley Public Service District (PSD) is located in the mountains of West Virginia. The PSD provides sewer service for a ski resort and the surrounding community. The local attractions drawing visitors meant that existing wastewater treatment system became inadequate in capacity and effluent quality. The sensitive nature of the environment and a discharge location into a small trout stream required a wastewater plant that produced consistent, high-quality effluent. After a long pre-design evaluation, membrane bioreactor (MBR) technology was selected as the preferred treatment process because of its ability to meet strict quality regulations.
The design of the two wastewater treatment plants in Canaan Valley (referred to as Zone C and Zone D plants) consisted of preliminary treatment (6-mm screening and grit removal), additional 2-mm fine screening, equalization basins, anoxic basins, pre-aeration basins, membrane reactor basins, return-activated sludge basin and pumping, waste-activated sludge basins, ultraviolet disinfection, post-aeration basin and effluent flow measurement.
The basis of the MBR design for the Canaan Valley WWTPs were hollow fiber membranes with a nominal pore size of 0.04 μ and an absolute pore size of 0.1 μ.
The project was advertised for bid in October 2009. A few weeks into the bidding period, a flat plate membrane manufacturer requested changes to the MBR specification to encourage competitive bidding. Because the project was partially funded by the American Recovery and Reinvestment Act of 2009 (ARRA), and competitive bidding was highly recommended, civil engineering firm Thrasher evaluated the changes requested by the flat plate membrane manufacturer. Thrasher issued a series of four addenda during the bidding process to include the flat plate membranes as an equal. In addition, the system warranty was modified to one year from substantial completion and five years of telephone support by the membrane manufacturer.
Bids were opened for the project on Nov. 19, 2009. The flat plate manufacturer was selected as the MBR supplier by the apparent low bidder. Four days after the bid opening, however, the MBR supplier attempted to withdraw its bid proposal due to pricing issues. Later it was learned that the pricing provided to the contractor was only for one of the two plants in the bid package. Multiple telephone conferences and meetings were conducted for three months following the bid opening. Thrasher worked with the contractor and MBR supplier to find money savings in the project to allow the project to move forward as bid.
At the completion of construction, the contractor conducted a clean water test of all the treatment basins at both WWTPs. The newly constructed plants passed the clean water testing required and were ready for startup in October 2011. The flat plate membrane supplier representatives came to the project site to oversee the startup process. During the site visit, the contractor was informed that wastewater could not be introduced to the membranes until the mixed liquor was a minimum concentration of 3,000 mg/L with at least 70% volatile solids in order to protect the membranes. The pore size for flat plate membranes is approximately
0.4 μ (significantly larger than hollow fiber membranes); thus, a biofilm on the membranes is required for proper operation.
Typically, activated sludge from a nearby WWTP is borrowed as seed to start up a the new plant. This was not a feasible option due to the lack of activated sludge plants nearby and the remote nature of the plant location. The sludge within the existing lagoons was deemed unacceptable by the membrane supplier. Also, dog food often is used during startup when the incoming flow is low strength, as a cheap supplemental carbon source to provide food for the microorganisms in the activated sludge. This traditional method was unacceptable for the two Canaan Valley MBRs due to high concentration of fats, oils and grease that could potentially clog the sensitive flat plate membranes.
The project team developed a systematic process to build biomass within a closed system (using the aerated equalization basin) using several hundred pounds of freeze-dried molasses, a colony of freeze-dried microorganisms, and, as a last resort, several truckloads of activated sludge transported a long distance to the remote site.
First, the aerated equalization basins were partially filled with incoming sewage. The sewage was aerated using evenly-spaced coarse-bubble diffusers. Freeze-dried molasses was added daily; the dosage was increased as the biomass grew within the equalization basin. In addition, a colony of freeze-dried microorganisms was purchased, incubated and added to the equalization basin. After a few weeks of moderate growth, a sludge truck was rented for a day to transport several loads of activated sludge to the plant (more than 60 miles each way).
Building biomass was required by the manufacturer for startup of the plant. It was not included in the specification, however, because it was originally written around the hollow fiber membrane manufacturer. Thus, the engineer and owner covered the bill for the time and supplies associated with building the biomass. After approximately five to six weeks, the mixed liquor suspended solids reached 5,000 mg/L and the project team was able to start up one train at each plant. The biomass continued to grow by treating influent wastewater and continuing to add supplemental carbon (freeze-dried molasses).
Both MBR plants have been fully operational since November 2011. To date, the permit limitations have not been exceeded and the plant staff has maintained an average mixed liquor suspended solids of 8,000 mg/L by continuing to add freeze-dried molasses during low loading conditions.
A poster presentation about the Canaan Valley PSD MBR plants was conducted at the 2014 Membrane Technology Conference in Las Vegas. The focus of the presentation was to make others aware of the issues that can arise when starting up an MBR plant in a remote location and the considerations designers should take during the planning stage of an MBR installation.