Wastewater Treatment

Village Creek is located near a residential area, and controlling odors generated by the plant is a high priority. Many of the treatment processes, such as bar screening and cyclone degritting, are enclosed in air-scrubbed buildings. The primary sludge thickeners, waste sludge dissolved air flotation thickeners (DAFT) and the weirs of the primary clarifiers are also covered and scrubbed. Village Creek uses both wet scrubbers and carbon adsorption scrubbers. Other odor control measures include the addition of chlorine to incoming wastewater, optimization of treatment processes and maintenance of good housekeeping around the plant.

After passing through bar screens, the wastewater goes to the primary clarifiers. In these large circular tanks, the wastewater flow is slowed to about one foot per minute so heavier solid materials can settle to the bottom. Grease and oil (scum) float to the surface and are removed and burned in an incinerator.

The solids that settle out (sludge) are scraped to the center of the clarifier (sedimentation tank ) and pumped to sludge degritting facilities for removal of sand and gravel. The bulk of the wastewater flows through the clarifier and is allowed to overflow the weirs for pumping to secondary treatment.

Village Creek uses conventional activated sludge as the heart of its treatment process. A biological treatment begins, which mimics the processes used by nature for purifying lakes and streams.

The process principle is to biologically convert pollutants that will not settle into substances that will settle. The wastewater is mixed with bacteria­ rich “activated sludge” in large aeration basins. Compressed air is fed through fine bubble diffusers to provide the bacteria and other microorganisms with enough oxygen to support the biological process in the wastewater. In fact, the bacteria “eat” organic matter in the wastewater. The process is controlled to minimize biological “burning up” of organic material.

Dissolved and suspended impurities in the wastewater are incorporated into the activated sludge floc through adsorption (when solids stick to the surface of the bacteria) and absorption (when dissolved gases and solids are taken into the bacteria where they can be assimilated) by the microorganisms.

Then the mixture of treated wastewater and activated sludge from the aeration basins is transferred to final clarifiers, where gravity separates the microorganisms from the wastewater. The clarified wastewater again overflows the clarifier weirs and moves on to the effluent filtration phase.

Most of the settled activated sludge is returned to the aeration basins to continue the treatment process. The remainder is pumped to Waste Sludge Concentration. Scum floating on the surface of the final clarifiers is removed and incinerated.

Anaerobic digesters provide an environment where anaerobic bacteria (bacteria that can’t live with oxygen present) are able to thrive and can break down the organics in sludge into stable compounds..

Anaerobic digestion reduces solids, odors and pathogens, and it conditions sludge so it dewaters rapidly. Methane gas, produced as a byproduct of this process, is used for mixing the digesters. This gas is recycled into fuel for incinerators, generating electricity and compressed air for the aeration basins.

The remaining solids suspended in the wastewater are removed by the passing of the wastewater through multimedia filters composed of gravel, sand and anthracite coal. Filters are cleaned periodically by “backwashing,” or sending clean water backward through the filter to flush out impurities. During backwash the bed “expands” and media particles bump against each other, allowing the particles to be washed away. Newer backwash filters use only sand and use a traveling bridge that continuously backwashes one small cell at a time.

Backwash water is moved to an equalization basin and fed at a constant rate to the backwash clarifier, where the solids settle out. Solids are transferred to Waste Sludge Concentration, as are solids from secondary treatment. Clarified backwash water is filtered again.

Waste activated sludge from the final clarifiers is concentrated by dissolved air flotation thickeners (DAFT). Compressed air is inserted into a mixture of water and the waste sludge. Small bubbles of oxygen form on the sludge particles, making them less dense than water and allowing the sludge to float to the surface of the thickener. The concentrated float sludge is removed by a skimmer and pumped to the digester area. The clarified liquid (subnate) is returned to the secondary treatment area.

After effluent filtration, the wastewater enters the chlorine contact basins, where it is mixed with chlorine and held for 20 minutes for disinfection to occur. The chlorine kills most of the disease-causing viruses and bacteria that remain.

However, since chlorine and its byproducts are toxic to aquatic life, sulfur dioxide is added after chlorination to remove the chlorine residuals. Sulfur dioxide dissolves to form sulfite, which reacts immediately with chlorine to form harmless chloride ions. After chlorination and dechlorination, the wastewater looks very much like drinking water and is discharged to the Trinity River.

The sludge that settles to the bottom of the primary clarifiers goes to this area. First, heavy inorganic particles such as sand and gravel (grit) are removed using cyclone degritters. The grit is classified (washed) and moved by conveyors to dump trucks and sent to a sanitary landfill.

After degritting, sludge flows to gravity thickeners to remove excess water. The liquid in the thickener overflows a weir and is returned to secondary treatment, and the thickened sludge is fed to the anaerobic digesters after being mixed with other sludge in a blend tank.