Flow enters the headworks through a 21â€ť gravity sewer. A Hycor HLC-300 spiral screen provides fine screening of the flow in the east 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 screen is provided in a second screenings channel. This channel contains a coarse manual bar screen and a Parkson mechanical bar screen. The screened wastewater enters the grit removal system. Grit removal is 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.
Biological Treatment System
Biological treatment is performed in the Conditioning (Selector) Tanks, consisting of Anaerobic Tanks and Anoxic Tanks, and the Aeration Basins. The design of the Biological Treatment System for the Liberty Lake WRF is for â€śBiological Nutrient Removal.â€ť Biological nutrient removal (BNR) removes nitrogen and phosphorus from the wastewater in addition to the traditional removal of BOD and TSS.
Screened and degritted sewage following discharge from the preliminary treatment system enters and Anaerobic Tank. These are operated in anaerobic conditions, defined as without any source of electron acceptor for oxidation of organics (BOD). Electron acceptors include dissolved oxygen, and other chemical compounds such a nitrates. Screened and degritted wastewater and return activated sludge from the clarifiers (RAS) are subjected to anaerobic conditions prior-to, following or concurrent with mixing together.
The wastewater and RAS mixture after anaerobic conditioning are discharged to the anoxic conditioning tanks. The Mixed liquor flows through these tanks in series. In the anoxic tanks the raw wastewater and the RAS mixture from the anaerobic tanks is combined with recirculated mixed liquor from the aeration basins. This mixture undergoes anoxic conditions where the nitrates and DO in the recirculated mixed liquor are reduced by the activated sludge biological organisms.
The anoxic basins discharge to the aeration basins. In the aeration basins mixed liquor is kept aerobic by air injection through the fine bubble air diffuser system. BOD removal and flocculation of suspended solids as well as nitrogen oxidation to nitrates takes place in the aeration basins.
Aeration basin â€śMixed liquorâ€ť is recirculated to the anoxic tanks where the anoxic reduction of nitrates by the biological organisms takes place. The nitrates are used as a substitute for oxygen in the oxidation of organic material.
The aeration basin effluent is diverted through a split box to the clarifiers. Solids settle in the clarifiers and are pumped back to the anaerobic zone to be mixed with the plantâ€™s influent. Excess solids (biological organism) growth are â€śwastedâ€ť to the sludge holding tanks, dewatered and disposed of off site.
Effluent from the clarifiers 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 â€śwastedâ€ť 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.