Plant Layout, Operation & Statistics
The graph below shows the number of gallons of pristine drinking water pumped (blue) to SunLand households, and the number of gallons of liquid returned to the sewage treatment plant (red) over the 13 years from 1998 through 2010.
Thus, we pump an average of about 95 Million gallons of water annually and have about 37 Million gallons returned to the treatment plant. That return averages just under 40% of outflow. The other 60% goes to irrigation, evaporation, and potentially, leaks in our system. Click on image to enlarge.
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The busy line graph below shows the number of gallons of pristine drinking water pumped to SunLand households by month over the 11 years from 2000 through 2010.
SunLand Wastewater Treatment Plant
Following is information about the operation of SunLand’s Sewage Treatment Plant.
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Advanced Treatment Technology;
A typical wastewater treatment plant will have aerobic oxidation and disinfection. SunLand has these plus the advanced (or tertiary) treatment of coagulation and filtration. These additional processes and a strict regimen of testing and reporting ensure our results meet mandated limits to classify SunLand as a class ‘A’ plant.
At many ‘plants’, treatment to a non-polluting, environmentally safe effluent before discharge (usually to a surface water of some sort) is the goal; The class ‘A’ water produced by the SunLand treatment plant is of such reliable, exceptional quality that it surpasses ‘adequate’ and becomes a valuable resource available for several beneficial uses which potentially could include augmenting SunLand golf course irrigation in the future, helping to conserve SunLand’s excellent pure drinking water reserves.
The sequence of treatment at the SunLand wastewater plant begins at the ‘headworks’ with a bar screen and ‘micro-strainer’ where undesirable objects in the incoming sewage (or influent) are retained and removed mechanically, followed by a grit chamber section facilitating gravity settling of grit, sand, egg shells, etc. Items such as dental floss, sanitary wipes and fine corrosives do not belong in any system and can cause expensive equipment to wear at accelerated rates.
Another item that is hard on all plants but difficult to remove is grease. Grease should go in your garbage. I would be re-miss to mention these items without mentioning the potential hazards of pharmaceuticals and personal care products (ppcp), particularly medicines; when all of these undesirable items go in the garbage (or drug take back program) instead of the sink or toilet, everybody benefits.
The primary effluent exiting the headworks, bar screen unit then goes into one of the two Sequencing Batch Reactor (SBR) tanks (or basins). An SBR has 5 specified cycles: Mix Fill, React Fill, React, Settle and Decant. The two tank design allows alternation so that as one basin may be settling and decanting, the other is busy mixing, filling and aerating. One basin must be available to accept influent (raw sewage) at all times because, though it is pumped intermittently to the plant, a minimum of approximately 100,000 gallons of influent arrives daily.
During Mix Fill, influent enters the basin and is mechanically mixed with a large volume of biomass (primarily bacteria plus protozoa and other invertebrates). This mostly aerobic microorganism community metabolizes, oxidizes or “reduces” organic and inorganic substances for life processes and is the ‘core’ of the activated sludge process. During the mix fill phase there is little or no dissolved oxygen present (anoxic), which aids in de-nitrification, phosphorous reduction and controlling filamentous bacteria (often a long, stringy bacteria that, if not controlled, can proliferate and hinder the settling process later).
Influent continues to enter the basin, but now 20 horsepower air blowers cycle on and off to create alternating aerobic and anoxic conditions. Many, many biological and chemical reactions of importance are occurring at almost all times, due to its prominence, I mention Total Nitrogen Removal as probably the most important goal of all wastewater treatment plants and is accomplished at the SunLand plant by alternating aerobic and anoxic conditions to facilitate Nitrification/De-nitrification. Most wastewater organisms are aerobes and need dissolved oxygen to oxidize substrates to obtain energy (cellular synthesis/respiration). Nitrifying bacteria which oxidize ammonia nitrogen (NH3) to nitrite (NO2) and then to nitrate (NO3) require oxygen. Denitrifying bacteria convert nitrate to nitrogen gas when oxygen is Not present, using the nitrate as an electron acceptor and harmlessly releasing the nitrogen gas into the air. The React Fill phase also has phosphorous uptake into the cell mass of the organisms and biochemical oxygen demand reduction.
Influent stops flowing into one basin and, by timing activated valves, switches to the other basin (which begins Mix Fill). The React phase allows the organisms to ‘work on’ the remaining nutrients and substrate for that particular batch. This is sometimes referred to as ‘polishing off’. At the end of this aerated period, the mixer stops and the aeration stops to settle the biomass.
Particles and organisms settle undisturbed to or near the bottom of the tank, leaving mostly clear, treated ‘water’ above the settled mass. (The other basin starts React Fill)
The treated ‘secondary’ effluent for that batch is then removed (decanted) from that basin and sent down the line for further treatment. The Decant stops at a preset limit in the basin so that the basin retains a mass of microorganisms in solution (74,000 gallons minimum) to treat the next ‘batch’ introduced, as the entire process begins again with Mix Fill.
At the end of each Decant cycle, before starting the whole cycle again with Mix Fill, each basin has a Waste Cycle; all wastewater plants accumulate ‘waste sludge’, mostly consisting of solids, non degradable constituents and excess organisms; and as the ‘bugs’ in the working biomass continue to multiply, their numbers must be monitored and kept in ‘check’ (we call it the food to mass ratio or F/M), otherwise you have a lot of full, inactive bugs that do not need to do much, as opposed to active, busy bugs that are ‘hungry’ for the next batch. This approximate 1% ‘trim’ each cycle is accomplished with the Waste Cycle.
The Waste Sludge goes to aerobic digesters (tanks with mechanical aerators) that continue to bio-degrade residuals, hopefully all the way to the endogenous phase (when organisms have degraded available food, then some of their own protoplasm, and finally cell lysis adds them to the mix to serve as food for other organisms). SunLand has minimal digester capacity, so degradation to these optimal levels isn’t really an option.
The aerobic digesters are similar to the basins in that they cycle on and off (more on than off) with aeration, but being filled with older, mostly oxidized wastes, have much longer settle times to separate the treated water from the biomass. The treated water obtained from these settle phases is put back into the plant at the headworks to be retreated. The remainder is classified as bio-solids. These bio-solids are often treated to valuable, beneficial uses, mostly as fertilizer and soil amendments that increase plant yields and health. SunLand is short on aerobic digestion tankage or ‘detention time’ and does not possess the equipment to further treat our bio-solids. We therefore truck by the gallon to the City of Sequim wastewater treatment plant, whom possess the equipment to further process bio-solids.
The Secondary Effluent that was separated from the biomass in the settle and decant phase takes a different route and is injected with a small amount of aluminum sulfate (alum) on it’s way to the flocculation tanks, filter and chlorine contact chamber. The flocculation tanks provide a propeller driven mixing environment that aids in the flocculation process. The alum helps flocculate or ‘gather together’ small or dissolved particles remaining in the effluent so that as the effluent arrives at the disc filter (tertiary/advanced treatment), the particles will more easily catch on the cloth material. Particles are mechanically removed from the filter and returned to headworks to be retreated. Water that passes through the filter continues to disinfection.
All wastewater treatment plants must disinfect the effluent prior to any discharge. No matter how well a facility treats the water, disinfection is required to eliminate any pathogens (disease or illness causing agents) that may remain. Disinfection is commonly done by exposure to Ultra Violet lighting systems, chlorine gas, sodium hypochlorite solutions or ozonation. Disinfection at SunLand WWTP is done with chlorine gas. By injecting the treated effluent with chlorine, then providing a sufficient time to mix in the chlorine contact chamber before discharge into a storage basin, disinfection is achieved.
The treated water is currently used to irrigate the spray field directly adjacent to the treatment plant, the grounds around the facility and as wash down/cleaning water to spray the various tanks and equipment that require it. We hope to use it for more beneficial uses in the future.
This has been a brief summary of the Wastewater Treatment Plant Process and Equipment. We are very proud of our plant facility and the results we achieve and are continuously trying to do an even better job.
If you would like to know more, we would be happy to show the facilities and equipment and answer any questions you may have.
Sunland Water District-Wastewater Treatment Plant
Willy Burbank-Superintendent of Water/Wastewater Facilities