What is Blending?

Blending is an operational strategy for managing high treatment plant flows which occur during major storm events due to infiltration and inflow (I & I) of rainwater into the sewer system. During  extreme wet weather events, wastewater becomes a small portion of the total  treatment plant flow.  Blending protects the treatment plant’s secondary (biological) treatment process from flows that would damage the process and/or exceed its design capacity.  During a blending event, all treatment plant flows undergo primary treatment, which consists of screening, grit removal, and chemically-aided removal of solids by settling.  Most of the flow then continues on to the secondary treatment process, where dissolved organic material is removed (oxidized) by bacteria and other microorganisms.  A portion of the flow is routed around (i.e. bypasses) secondary treatment, and then recombines with the main flow prior to disinfection.  When blending, all flows undergo disinfection, and the treatment plant is still required to meet all applicable regulatory limits in its discharge permit.

Blending is used at the LGVSD plant when plant flows (which are typically in the range of 2-4 million gallons per day in the winter) exceed 8 million gallons per day.  Blending usually lasts for between 2-8 hours, but can persist for a day or more during extreme storm events.  Blending events, including duration and volume, are reported below.

What Can Be Done to Reduce the Need for Blending?

High wet weather flows to the treatment plant occur as a result of rainwater entering the sewer system.  Much of this occurs underground in older sewer mains and manholes, which are District-owned, and in sewer laterals, which are privately owned.  Property owners have a responsibility to ensure that their sewer laterals are in good working order.  Most of the sewer laterals have not been replaced and are very old and leaky.  They are a large part of the problem which ultimately increases sewer rates.  The District has a Sewer Lateral Assistance  program to help property owners finance the replacement of their sewer lateral.  Property owners should also make sure that their on-site storm drainage systems (roof, surface, and foundation drains) and swimming pools are not connected to the sewer system.  The District has a continuous program for replacing old sewer mains and other sewer infrastructure, funded by a portion of the fees that customers pay to use the sewer system.  The District is also in the process of designing improvements to upgrade aging equipment and treatment processes to accommodate higher flows during wet weather events.