Challenge

In 2004, a U.S. District Court decision designated the effluent pond for the wastewater treatment plant in Healdsburg, California, a “waters of the United States,” making the city accountable to stringent new effluent requirements. Prior to the ruling, the city had been using aerated lagoons that discharged into an inactive gravel mining pond.

Solution

To meet the stricter NPDES permit requirements, as well as to gain the ability to treat reclaimed water for irrigation, Healdsburg chose a Siemens membrane bioreactor system (MBR) combined with Cannibal® solids reduction process to create an advanced and innovative new plant that had the greatest likelihood of meeting future regulations.

The city also hoped this technology combination would quickly increase the number of operating MBR facilities, extending its base of experience for these kinds of plants. Designed for an average dry weather flow of 1.6 MGD with a sustained peak of 4 MGD, the new plant was commissioned in May 2008, with startup of the solids reduction process later that year.

The MBR system consists of:

• Two parallel biological trains configured in a Modified Ludzack-Ettinger (MLE) process for biological nutrient removal, and 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. 
• Filtrate is drawn through the membranes by applying a suction pressure from a rotary-lobe positive displacement pump, while 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 ultraviolet (UV) lamps and then to discharge or reclamation.

Results

By combining these technologies, the city will prolong membrane life and expects a projected yield that will result in significant savings in sludge handling and hauling costs. One of the old treatment lagoons is now used as an equalization basin during winter storm events, which allowed the city to build the treatment plant with a smaller peak flow than otherwise needed.

Currently, the Healdsburg plant is exceeding the water quality effluent limits in its NPDES permit, and it's achieving the lowest effluent copper levels (less than 0.01 mg/L) in the region. The city is able to market and recycle its high-quality effluent for beneficial reuse and has plans for unrestricted irrigation use in the future.