Flow enters the headworks through a 21” gravity sewer. A Lakeside Raptor Micro Strainer provides fine screening of the flow in the west channel. The screenings are washed and pressed prior to disposal in a dumpster. The screenings are removed by a licensed solid waste handler and transported to the Valley regional transfer facility. Redundancy for the Lakeside screen is provided in a second screenings channel. This channel contains a Hycor HLC-300 spiral screen. The screened wastewater enters the grit removal system. Grit, sand and other small particles are removed through two parallel grit channels, designed to operate one at a time, allowing the other channel to be taken out of service for cleaning. An outlet weir in each channel controls velocity in the grit channels and meters the flow as water moves into the equalization (EQ) basin. The EQ basin is utilized to smooth out the daily diurnal flows coming into the Water Reclamation Facility.
BIOLOGICAL TREATMENT SYSTEM
Wastewater is treated through several different processes including physical, chemical and biological processes to remove the solids and separate the water. The Liberty Lake WRF was designed as a Biological Nutrient Removal (BNR) process in order to remove phosphorus and nitrogen from the wastewater in addition to TSS and BOD. A typical BNR process will consist of anaerobic, anoxic and aerobic basins.
The screened and de-gritted wastewater is mixed with Return Activated Sludge (RAS) from the secondary clarifiers as it flows from the EQ basin into the anaerobic basin. An anaerobic basin is one that contains no molecular oxygen or oxidized nitrogen species such as nitrite (NO2) or nitrate (NO3) and thus does not have any source of final electron acceptor for the oxidation of organics (BOD). One of the main goals of the anaerobic basin is to provide a set of conditions that help select for the growth of phosphorus accumulating bacteria that will eventually lend a hand in removing phosphorus from the wastewater.
After conditioning in the anaerobic basin, the wastewater is discharged to the Anoxic basin for further treatment. An Anoxic basin is one that contains little to no molecular oxygen but does contain chemically bound oxygen in the form of nitrate (NO3). The wastewater in the anoxic basin is combined with recirculated mixed liquor from the aeration basin and undergoes a process called denitrification. Here, the activated sludge microorganisms biologically reduce the nitrates to nitrogen gas, forming bubbles that rise to the surface of the basin and escape into the atmosphere. The nitrates are used as a substitute for oxygen in the oxidation of organic material.
The anoxic basins are discharged into the aerobic basin where the mixed liquor (the entire community of microorganisms) is kept under aerobic conditions by air injection through fine bubble diffusers. The organisms are able to remove soluble food such as organics (BOD) and other nutrients by converting these chemicals present in the wastewater into new cellular material via cellular reproduction. Two additional important processes occur in the aeration basin. The first is the process of nitrification in which ammonia is biologically converted into nitrate (this mixed liquor is then recirculated back to the anoxic basin for denitrification). The second process is the flocculation of the mixed liquor which will help it “stick together” and settle out in the clarifiers.
The aeration basin effluent is diverted through a split box to two secondary clarifiers. Solids (biological organisms) that settle in the clarifiers will take one of two paths. Some solids called Return Activated Sludge (RAS) are pumped back to the anaerobic zone to be mixed with the plant’s influent and serve as a source of seed bacteria for the incoming food (BOD). Excess solids called Waste Activated Sludge (WAS) in the bottom of the clarifier are pumped to the sludge holding tanks, dewatered and disposed of off-site.
TERTIARY MEMBRANE FILTRATION
Effluent from the secondary clarifiers flows into the membrane filtration building. The ultrafiltration membranes are Suez ZeeWeed 500D with a 0.04 um pore size. These filtration membranes are able to remove most bacteria and some viruses, suspended solids, colloidal materials and some large molecules. The membranes are periodically backwashed and cleaned with all water used in cleaning returned back to the head of the plant for treatment.
Effluent from the tertiary membrane filtration system then is directed to U/V basins, where there are 5 vertical, 40 lamp, low pressure ultraviolet disinfection modules. Flow is directed through these modules and exposed to ultraviolet light which disrupts biological organisms. Channels are configured in such a way that flow can be exposed to 2, 3, or 5 modules as it passes through these channels.
The solids “wasted” from the secondary clarifiers are held in the sludge storage tanks. The contents of these tanks are mixed and aerated by coarse bubble diffusers; detention time is such that secondary phosphorus release is avoided. Solids are then sent to the belt filter press for dewatering. The press is an Andritz 2 meter unit. Polymer is added to the solids and ran across the press. Percent solids coming off the press run an average of 15 to 16% dry solids. This material is deposited in a trailer and then transported and applied to dry land farms by Boulder Park Inc. in Central Washington. BPI is permitted by the Department of Ecology as a “Beneficial Use Facility”. All biosolids at this site must meet EPA 503 regulations for metals and nutrients. Prior to application, all application site soils are analyzed to determine maximum agronomic loads so biosolids are applied to maintain levels below determined levels. The University of Washington is actively engaged in research in studying the numerous benefits of biosolids vs. crops grown with conventional fertilizers/herbicides. Overall health and crop yields are much improved by this beneficial use of biosolids.