The City of Redlands selected ZeeWeed® MBR as the best technology to produce recycled wastewater for the new Mountainview Power Station and other reuse applications.
Click here to enlarge imageThe membranes are immersed directly into the mixed liquor and draw treated effluent into the fiber using a gentle suction. A ZeeWeed® membrane fiber acts as a physical barrier, preventing suspended solids and pathogens from entering into the final effluent.
Since sludge settling is not required, MBRs can operate at much higher mixed liquor suspended solids (MLSS) concentrations, typically in the range of 8,000 to 15,000 mg/L, enhancing the system’s ability to remove nitrogen and enabling it to operate in a relatively small bioreactor volume.
At Redlands, side screening of mixed liquor, with 0.75 inch internal feed rotary drum screens, protects the membranes from finer materials that passed through the pretreatment system and could cause damage. Treated effluent is collected in a common header pipe and flows to a chlorine contact disinfection system before being pumped to the power plant.
The simple design of an MBR means that complete wastewater treatment can be carried out in one or two steps which significantly reduces the footprint of the plant and the operational equipment. Main equipment for a typical MBR plant includes a fine screen, membrane cassettes, a bioreactor, permeate pumps, blowers for process and membrane scouring, and clean-in-place equipment to handle membrane backpulsing and washing.
The backpulsing process forces permeate water back through the membranes. This dislodges any particles that may adhere to the membranes. Intermittent aeration of the membranes is also used to scour debris from the fibers and provides mixing within the process tank to maintain solids in suspension. When necessary, in-situ chemical cleaning can be automatically performed if membrane fouling reduces permeability below a specified performance level.
Turbidity meters ensure that each train meets turbidity requirements for Title 22 compliance (less than 0.2 NTU 95 percent of the time) and also indirectly verifies the integrity of the membranes. In the unlikely event that turbidity rises, an alarm will notify the operator and the appropriate action can be taken.
Conclusion
The 20-year deal with the power plant will bring in about $500,000 annually for the City of Redlands, and Headrick is always on the lookout for new customers to buy the city’s recycled water. The 14,000-foot distribution line that serves the power plant is equipped with many turnouts to serve other potential customers along the way. Currently the surplus reuse water is sent to percolation ponds to slowly trickle back into the water table, but Headrick hopes to eventually sell recycled water for irrigation at orange groves, golf courses, city property and also for dust suppression at the neighboring landfill site.
About the Authors: Contributors to this article included Dave Commons, Water Operations Manager, City of Redlands; George Beliew, PE, Partner, Carollo Engineers; Sava S. Nedic, PE, Principal Project Manager, CH2M HILL; and Jodi Cumin, P.Eng., Regional Manager, Zenon Environmental Inc.