Treatment Process

Where it all begins - flush it and forget it? I think not! You would be surprised how much treatment is involved for wastewater.

There are several stages of treatment--first let’s start in the collection system.

Collections System  

There are approximately 120 miles of sewer lines in Hays, mostly running in the alleyways throughout Hays.  All household and business toilets, baths, showers, and most floor drains are connected to the sanitary sewer system.  All of those miles of sewer line pipe end up at the Hays Water Reclamation & Reuse Facility (WRRF).

Preliminary Treatment

The first stage of treatment at the WRRF is preliminary treatment.  These systems are mostly physical in nature.  These processes facilitate operations and protect downstream equipment.
First, the wastewater enters a rotary drum screen.  This rotating drum has ¼” holes that trap debris.  The drum screen removes all of the material that shouldn’t be in the wastewater stream. This collected debris is called screenings.  It is collected in a dumpster and disposed of in the landfill.  Examples of items typically found in wastewater screenings: rags, “flushable” wipes, rope, toys, feminine hygiene products, prophylactics, and grease.
The next part of preliminary treatment is grit removal.  Some examples of grit are sand, eggshells, and coffee grounds.  If left in the system, grit has the potential to fill downstream basins and cause premature wear on pumps, mixers and other equipment.  Grit is collected in a dumpster and disposed of in the landfill.

Secondary Treatment

The magic of a wastewater treatment system is the biological treatment, and it happens in the secondary treatment train.  Here, the wastewater is given air, mixing, and time.  The activated sludge process provides the biological treatment of the wastewater to reduce Biological Oxygen Demand (BOD), Total Suspended Solids (TSS), and reduce nitrogen via nitrification and denitrification.

Wastewater and Return Activated Sludge (RAS) flow into the 185’, 4.5-million gallon aeration basin. Treatment is accomplished within the completely mixed basin by alternating aerobic (with air) and anoxic (without air) cycles.  These cycles occur for sufficient periods of time to facilitate BOD and TSS reduction, convert ammonia to nitrate via the nitrification cycle, and then convert nitrate to nitrogen gas (or denitrification).  During the anoxic process, some microorganisms are also busy taking up phosphorus from the wastewater in a process called luxury phosphorus uptake.

After leaving the aeration basin, the water flows to one of two final clarifiers.  These clarifiers provide a calm, quiescent area that permits the activated sludge solids to settle.  Clarified wastewater passes over the weirs and on to the next treatment step.  Activated sludge solids are settled.  A portion of these settled solids are returned with raw wastewater to the aeration basin and is called Return Activated Sludge, or RAS.  Conversely, the remaining portion of settled solids are wasted to the aerobic digesters and is called Waste Activated Sludge, or WAS.  WRRF staff determines how much to waste and how much to return using a common wastewater metric called food to microorganism ratio, or F:M for short.

Tertiary Filtration

The tertiary disc filter receives effluent from the final clarifiers and removes any suspended solids that may not have settled in the clarifiers.  This level of filtration allows us to achieve the level of solids removal necessary for water reuse. 

Disinfection

The next step is disinfection.  Effluent from the tertiary disc filter is directed to one of three ultraviolet (UV) disinfection modules.  UV disinfection uses ultraviolet light to disturb and damage the genetic material in microorganisms.  A fourth UV module is larger, provides a larger dose of UV light, and is dedicated for reuse water.  Effluent flows out of the UV modules and into the effluent pump station for conveyance to either the reuse system via pumping or final discharge to Chetolah Creek via gravity.

Plant Effluent & Reuse

Treated plant effluent is reclaimed for WRRF facility process water, non-potable uses for commercial businesses, and irrigation water.  Currently, the City utilizes approximately 25% of its wastewater effluent beneficially for irrigation water at the Bickle-Schmidt Sports Complex, Hays Municipal Golf Course, Larks Park, FHSU Track & Field complex, FHSU soccer complex, and the Dusty Glassman ballfields.  Without water reuse, these types of facilities may not exist in our area due to water restraints.

Sludge Digestion

Waste Activated Sludge (WAS) is processed through two Aerobic Digesters to provide solids reduction and stabilization prior to disposal.  The Aerobic Digesters are designed to produce a Class B biosolids for land application. 

Dewatering

Sludge that is processed through the Aerobic Digestion system is ultimately disposed of offsite. Following digestion, the stabilized biosolids are approximately 1.5% solids.  The biosolids are then transferred to the dewatering centrifuge.  The centrifuge removes as much excess water as possible from the biosolids.  The product leaving the centrifuge is approximately 20% “cake” biosolids.  The dewatered cake is discharged from the centrifuge to a conveyor, which is stored onsite at the WRRF for application as fertilizer on area farm fields.