Wastewater treatment in San Diego

At first, Hollywood and wastewater treatment may seem like an unlikely pair. But the City of San Diego Metropolitan Wastewater Department’s (MWWD) Metro Biosolids Center (MBC) is an uncommon place.

From serving as a two-time movie set to winning awards for striking architectural design, the MBC has been recognized as an industry leader in creative thinking, community involvement and environmental responsibility.

The MWWD includes several facilities interconnected through an elaborate pipeline system, including the MBC, the Point Loma Wastewater Treatment Plant and the North City Water Reclamation Plant.

Completed in 1998, the MBC was designed with an artistic flair not commonly associated with government departments. Older MWWD buildings have been renovated to reflect a similar sensibility, integrating the buildings into the communities they serve.

A real powerhouse

The MWWD’s real strength extends far beyond its facilities’ good looks. The MWWD serves a 450- square-mile area in Greater San Diego. Two million people from 16 cities and districts generate up to 680,000 cubic metres (180 million gallons) of wastewater daily. The plant handles this challenge with ease. In the process, it not only generates enough power to be entirely self-sufficient, but excess power is sold to the local energy grid, offsetting operating costs and making a positive environmental impact on the community.

The heart of the MWWD system, the MBC, processes both raw sludge from North City and digested sludge from the Point Loma plant. The digested solids arrive via a 17- mile pipeline.

This “offsite” solids treatment is fairly rare in the industry and required extensive planning. In fact, the MWWD invested over 33 million U.S. dollars in engineering and construction management costs for the MBC development.

Keeping optimum temperature

Raw sludge is treated at both MBC and Point Loma using the same process. After degritting and screening to eliminate large materials, raw solids are directed to chambers to remove heavy inorganic particles and organic solids. Ferric chloride and organic polymers are added to help the waste particles bond together and settle out.

At this point in the process, approximately 80 percent of the total suspended solids in the water have been removed, and the treated wastewater, or effluent, can be discharged to the ocean. The sludge continues on for additional processing.

Alfa Laval spiral heat exchangers are used to preheat the sludge. In any given minute, 2,500–3,000 litres (650–780 gallons) of sludge flow through the spirals.

The undigested sludge from the North City plant is pumped into one of the MBC digesters. A heat and bacterial process similar to human digestion reduces the volume dramatically. Over 15 to 20 days, bacteria consume approximately 60 percent of organic matter, reducing volatile solids and creating digested sludge in the process.

The spiral heat exchangers also maintain the heat in the airtight digesters, keeping the sludge at the optimum temperature for digestion.

The spiral heat exchangers are easier to maintain, are more efficient and have a smaller footprint than the shell-and-tube technology commonly used in the past.

Reduced disposal costs

After digestion, sludge from both the Point Loma and North City facilities is commingled at the MBC and it all moves on for processing in Alfa Laval decanter centrifuges. These decanters dewater the blended wet bio solids by using centrifugal.

Thed decanters dewater the blended wet biosolids.As the decanter rotates, the liquid, or centrate, is separated from the dewatered biosolids. The centrate returns through the sewer system to Point Loma for processing. The pumps feeding the decanters automatically adjust the speed, depending on programmed parameters and the number of centrifuges on line. The remaining biosolids, or cake, are the consistency of wet clay, which is transported to nearby landfills or land applied for beneficial use. The decanters help produce a high quality cake; MBC reduced its annual disposal costs by 1.8 million dollars thanks to the improved cake quality produced by the decanters.

The power of self-sufficiency

The digestion process and liquid biosolids storage tanks both create methane gas as a by-product. This is captured to produce electricity to meet the facilities’ power needs, and the surplus powers other homes in the community. The MBC saves the city approximately 45,000 U.S. dollars a week by generating its own power. Even when the city of San Diego was entangled in an energy crisis in 2002, all MWWD facilities remained operational during brownouts.

For all of its environmental efforts, the MWWD was the first publicly owned wastewater treatment operation in the USA to receive ISO 14001 Certification from the International Organization for Standardization in Geneva, Switzerland. This award is a testament to the department’s unwavering respect for the environment.

Up and running

With the combination of quality equipment, self-sufficient power, strong internal resources and an ongoing Performance Agreement with Alfa Laval, downtime is minimized or often eliminated – allowing the system to meet the stringent standards of the Environmental Protection Agency (EPA).

While internal maintenance personnel do most of the daily maintenance, the Alfa Laval Performance Agreement provides routine equipment maintenance, technical support and onsite training.

The result: a system as dependable as it is striking – one that’s picture perfect.

Alfa Laval / MWWD cooperation

In wastewater treatment, downtime is unacceptable – equipment downtime can quickly lead to harmful environmental consequences. In this environment, preventative maintenance is key.

Preventing downtime led the Metropolitan Wastewater Department to choose a Performance Agreement with Alfa Laval for equipment maintenance at the Metro Biosolids Center.

The Alfa Laval Performance Agreement pertains to eight DS-706 dewatering decanters and five PM-95000 thickening decanters. (The Alfa Laval spiral heat exchangers on-site are not part of the agreement, as they are virtually maintenance free.)

Routine equipment maintenance and inspection at regular intervals; minor, intermediate and major service over the lifecycle of the equipment;labour and part expenses; telephone technical support; onsite training; and the extended warranty are all rolled into one fixed cost. This program allows accurate budget planning and eliminates unpleasant financial surprises if a technical need arises.

In between visits, MBC’s internal staff complete regular maintenance. They have ongoing access to technical resources at Alfa Laval’s Contact Center via phone. Plus, onsite training for new staff ensures that the staff are trained in all proper maintenance procedures.

This Performance Agreement has been in effect since 2001 and is scheduled to run until 2005, with a renewal at the end of each 12-month period.

Jack Swerlein, Plant Superintendent at the MWWD, says: “We have saved 1.8 million dollars on our annual biosolid disposal costs after changing our dewatering process from a belt press operation to Alfa Laval centrifuges. The Performance Agreement and  Extended Warranty have played a key role in achieving these cost savings".