Introduction to Sewage Treatment

The communities on Donner Summit produce a lot of sewage and the growth of the communities will only produce more.  Today sewage flow averages 200k gallons per day and the plant’s State set capacity is 520k gallons per day.  An average December day may see 275k  gallons of influent and a summer day 150k gallons.  Spring flows are the highest because of I and I (inflow and infiltration ñ leaks).  Fall flows are the lowest: 90-100k gallons per day.

Sewage comes to the DSPUD plant and enters the 200k gallon equalization tank outside the fire station (link picture to the right).  The tank evens out the flows to the plant so that a constant processing rate can be maintained.  The tank fills during the day and drops at night.  It averages about 20% full but on a heavy weekends it may be 70% full.

eualizer Prior to entering the tank, a bar screen removes inorganic materials and washes and breaks down the organics.  A communator breaks up the organics for more efficient processing.  Influent is also sampled at this stage and one liter samples are sent away for analysis of incoming sewage.

After leaving the equalization tank, soda ash is injected to raise the Ph of the water becaus alkalinity is needed for bacterial to grow.  The soda ash silo is part of the recent upgrade and holds 42 tons of soda ash.  It batches and feeds the soda ash into the pipeline leading to the two package plants.  The next upgrade to the plant will include remote control and remote monitoring of the soda ash injection process.

The processing of sewage requires bacteria that break down the ammonia.  It is necessary to maintain the proper balance of bacteria to solids (BOD’s or biochemical oxygen demand) in the treatment process. In order to have the proper amount of bacteria on hand for the winter months, extra bacteria need to be grown in the warm summer months.  When winter rolls around, the bacteria have trouble growing.  In order to guarantee the proper bacteria count, ammonia is actually added to the summer sewage to promote bacteria growth. 

An interesting sidelight here is that the DSPUD may contract with the Jelly Belly factory in Fairfield for some of their waste which is high in sugar and will promote bacterial growth.
There are two separate package plants, the round structures, which do the initial stages of sewage processing.  They are alike except in size and both have been recently upgraded with the larger one’s upgrading having been completed in November of 2006.  It’s capacity was increased by almost 1/3.

Each plant consists of an outer ring and an inner center section.  Microorganisms are in the outer ring which are full of mixed liquor suspended solids (MLSS).

The “mixed liquor” in the outer ring is a combination of fresh sewage and sewage which has been partially fixed by bacteria.   The fixing  is done as sewage passes through webs which are 12í X 6í X 4í  filter like structures consisting of multiple layers of mesh to which bacteria affix themselves to break down the sewage.  This break down consists of changing the ammonia in the sewage water to nitrites and nitrates.   Since the product of the sewer plant ends up, during at least some of the year in the Yuba River and since nitrates are a fertilizer that will cause eutrophication (algae growth, sediment, and oxygen depletion), the nitrates and nitrites need to be converted as well.filterplant

Go on the page two of the field trip.