Wastewater Process Summary
Screening. Screening is the process of removing large material from the wastewater as it enters the beginning of the wastewater treatment process. When wastewater arrives at the treatment plant, it contains a large amount of debris that gets washed off the streets and into catch basins during rainstorms. The mechanical bar-screen at the treatment facility removes this material which usually consists of rags, sticks, litter, leaves, and just about anything else that can find its way into a catch basin during a storm.
- Bar-Screens. La Porte's screening system consists of two centenary bar-screens. This allows one to be used as a backup bar-screen if there is trouble with the primary bar-screen.
Grit Removal. Grit is the material in the wastewater that is composed of small, inorganic solids. Material like coffee grounds, sand, eggshells, and gravel. This material, like the larger screenings which were removed prior to the process, can not be treated by the wastewater treatment process and can cause excessive wear on the pumps and settling tank drags.
- Removal. The grit is removed in two long narrow channels that have a "scraper" that drags the grit from the bottom of the channel where the grit settles out, back to an auger where it is lifted out of the channel and deposited into a holding container.
- Separation. After the inorganic grit is removed from the wastewater, the next step is to remove the organic solids. Organic solids are materials that can be processed at the treatment facility through the anaerobic digestion process. To separate these solids from the rest of the wastewater we use simple gravity.
- Tanks. The wastewater is split into four long tanks that allow the water to slow down to less than 0.5 feet per second. At this speed, floatable material will separate from the water to the top, and the heavier solid material will sink to the bottom. The floatable material (also called "scum") is skimmed off the surface of the water and also sent to the digester.
Secondary Treatment. The secondary treatment stage follows primary settling. During secondary treatment, the wastewater is pumped to one of two treatment units.
- Rotary Trickling Filter Unit. One of the units is a large circular distributor that moves and sprays the water over layers of plastic media. This unit is called a "rotary trickling filter". The wastewater that is sprayed on the plastic media then "trickles down" (hence the name) the plastic to an underdrain system at the bottom where the water is collected, then transferred. While the water is trickling down the plastic media, bacteria that are growing on the media "eat" the leftover dissolved organic material and nutrients that are in the water.
Secondary Settling. When the wastewater is processed through the second stage of treatment, the bacteria that remove the nutrients from the wastewater die over time and enter the waste stream as a solid material. Just like with the primary settling stage, this material can then be settled out of the wastewater through additional settling.
- Tanks. The secondary settling tanks are very similar to the primary settling tanks. The secondary settling tanks also have a series of scrapers in the bottom that drag the settling solid material to a sump at the end of the tank where it can be pumped out. The solids that are captured during this process are also sent to the anaerobic digester system so they can be reclaimed. Unlike the primary settling system, instead of four tanks, there are only two as the volume of solids that can be expected to be captured here are considerably less than in the primary settling tanks.
Advanced Secondary Treatment. Advanced secondary treatment begins at the end of the secondary settling stage. The wastewater at this point is quite clean compared to the state that it arrived in at the treatment facility. One of the last remaining concerns is the amount of ammonia in the wastewater.
- Biotower. Ammonia at higher levels is toxic to aquatic life and can result in harm to the environment. To remove the ammonia from the wastewater, La Porte's treatment facility involves a special type of trickling filter called a biotower. The biotowers function similar to a regular tricking filter, except that a biotower has 20 feet of media, and a regular trickling filter has about 5 feet.
Advanced Secondary Setting.. After the wastewater has been through advanced secondary treatment, the waste stream is once again full of organic material from the bacteria that has sloughed from the two bio towers. At this stage, the remaining solid material is in very small particles, which do not settle as easily as large clumps of material would.
- Process Time. For this reason, the capacity of the advanced secondary settling tanks is more than the volume of the primary and secondary settling tanks together. This allows the wastewater the maximum amount of time to slow down and allows the solids to settle out.
Disinfection. After completing the tertiary settling stage, the wastewater is almost ready to be returned back into the environment. Up to this point, the main thrust of treatment has been with removing pollutants like solids, ammonia, and oxygen demand that can cause harm to the aquatic life downstream.
Since the treatment process uses naturally occurring bacteria to aid in the treatment there have been no steps taken to eliminate those bacteria. The disinfection process is the final treatment stage whose sole purpose is to eliminate the bacteria that we have depended upon so far.
Anaerobic Digestion. After all the solids from the various settling tanks are collected, they are pumped into primary digester #1. The reason that the process is called anaerobic is that the entire process takes place in the absence of oxygen. The La Porte Wastewater Treatment Facility has three digesters. Two primary digesters and a secondary digester. The two primary digesters are heated to 95 degrees Fahrenheit and mixed, where the secondary digester is unheated and is not mixed.
Get details about the anaerobic digestion stage of the overall treatment process.
Storage & Land Application. After the solids have been treated in the anaerobic digesters, they are pumped to a storage tank where they are held for six months. During this time, the treated solids are allowed to further settle and the water is decanted from the top, which helps thicken the remaining material.
Process Control & Maintenance
- The Wastewater Treatment Facility is a complex process involving dozens of pumps, hundreds of valves, and millions of gallons of wastewater. All this process is monitored and controlled by a centralized computer-based network.
- The operator on duty can monitor which pumps are running, how much water they are pumping, which valves are open or closed, how much chlorine is being applied, and even which lift stations in the city are currently pumping. By recording this information in databases, the operating staff can then analyze it and determine which is the best way to optimize treatment in the facility for any given situation.
Maintenance Facilities. With all these complex systems and moving parts, there is a lot that can go wrong. It's the job of the maintenance crew to fix the problems when they happen and to anticipate future problems before they occur. The maintenance crew handles everything from rebuilding a broken pump or replacing a valve to fashioning new replacement parts and mowing the grass.
In any complex system, no matter how well controlled it is if there are poor maintenance things will grind to a halt fairly quickly.