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When the city of Healdsburg, Calif., needed to upgrade its wastewater treatment facility to meet Clean Water Act requirements and a National Pollutant Discharge Elimination System (NPDES) permit, it chose MBR treatment technology. This enabled the city to eventually reclaim water for unrestricted irrigation use.
In 2004, a U.S. District Court decision designated the Healdsburg plant’s effluent pond as “waters of the U.S.” Under regulations adopted by the state regulatory agency for discharge to such waters, this meant that the plant needed to meet more stringent effluent requirements. The plant had been using aerated lagoons that discharged into an inactive gravel mining pond since 1970.
To meet the strict requirements and be able to treat reclaimed water for irrigation, the city investigated two options: a conventional biological nutrient removal (BNR) plant with tertiary treatment and a membrane bioreactor (MBR) system combined with a solids reduction process.
Healdsburg ultimately chose an MBR system and solids reduction process for the following reasons:
Commissioned in May 2008, the 4-million-gal-per-day (mgd) plant consists of a Memjet MBR system and the Cannibal solids reduction process. It is designed for an average dry-weather flow of 1.6 mgd, with a sustained peak of 4 mgd. One of the old treatment lagoons is now used as an equalization basin during winter storm events. This allowed the city to build the treatment plant with a smaller peak flow than it would have otherwise needed to accommodate peak flows. The equalization volume is designed for a one-in-100-year storm flow.
The MBR system consists of two parallel biological trains configured in a Modified Ludzack-Ettinger (MLE) process (Figure 1) for biological nutrient removal, plus five membrane tanks with a common overflow weir. Each membrane tank is equipped with 144 immersed membrane modules, with additional space for up to 160 modules.
The membrane operating system was designed so that one tank can be taken out of service in all flow conditions for maintenance or cleaning. Filtrate is drawn through the membranes by applying a suction pressure from a rotary-lobe positive displacement pump, while the suspended solids and bacteria are retained in the mixed liquor and overflowed back into the biological system.
The filtrate flows through low-pressure, high-intensity UV lamps and then to discharge. At some point in the future, the plant effluent will be reclaimed for unrestricted use.
By combining the solids reduction process with the MBR, the city expects to prolong membrane life because of the solids reduction process enhanced solids removal. The facility uses fine screens (<250 um) to remove solids which can damage the membrane; their accumulation decreases the effective membrane surface area.
The solids reduction process started up in December 2008, and the city expects to realize a 50% to 75% reduction in biosolids yield. The projected yield will result in significant savings in sludge handling and hauling costs. To date, the Healdsburg plant is exceeding the water quality effluent limits in its NPDES permit and achieving very low effluent copper levels (<0.01 mg/L) in the region.
“The MBR/solids reduction system has well positioned the city to market and recycle its high-quality effluent for beneficial reuse,” said Jim Flugum, senior civil engineer at the city of Healdsburg Public Works Department, “and will allow us to meet more stringent water quality regulations in the future.”