1.4: Preliminary Treatment
Learning Outcomes
- Understand why preliminary treatment is necessary
- Compare and contrast the different types of preliminary treatment methods
- Evaluate the benefits of different treatment methods
Nuisance Substances
Preliminary treatment is the first step in treating raw wastewater. When the wastewater first enters the wastewater treatment facility there are a lot of nuisance materials that have found their way into the collection system. Items such as large rags, bottles, tree branches, and numerous other nuisance items can be found in the influent to the treatment facility. These large items can cause damage to downstream pumps, take up valuable space in settling tanks, and can be hazardous to other mechanical equipment needed in the treatment process. So the first, or preliminary step is to remove these large items.
Screening
One method to remove large debris items is by using a bar screen . Bar screens are capable of removing items that are larger than the spacing between the bars. For example, a ¾” bar screen will hold back any debris that is larger than ¾” and anything smaller will pass through it. Common items that are removed in this preliminary treatment step are rags, roots, large rocks and aggregate, bottles, cans, and numerous other large objects that can make their way into the wastewater collection system.
Over time, the debris collected behind the bar screen will need to be removed. This is done either manually or automatically. Manual bar screens typically have large spacing, 2” to 4” is common, between the bars. To manually clean a bar screen an operator will use a rake and pull out all of the debris stuck behind the bar screen. This can be extremely laborious and dangerous as the operator is lifting heavy objects over an open trench. Ideally, a wastewater treatment plant will have two or more channels with bar screens. This way one channel can be isolated and the debris can be removed without having wastewater flowing through it. Although this is safer, it is still laborious.
Newer bar screens are raked automatically. On more sophisticated systems, the channel where the bar screen is installed will have an upstream and downstream level sensor. When the debris is blocked by the bar screen it will also inhibit the flow of wastewater through the screen. This will cause the upstream water level to rise. When the difference between the upstream and downstream levels reaches a predetermined setpoint, the motorized rake will swing down and automatically pull the debris out of the channel and into a hopper. Other systems will automatically initiate the raking sequence based on a timer or will just run continuously.
Most automatic bar screens will have a washer/compactor that receives the removed debris. The washer/compactor will wash the debris removing any of the organic matter and then compact it. Washing is important because in this preliminary step we are only trying to remove the large inorganic items. We want to keep the organic material in the wastewater stream as it will be utilized in subsequent treatment processes. Compacting the debris is critical as it removes most of the water and makes it easier to transport for disposal.
Since automatic bar screens can run autonomously, the bar spacing can be smaller. Most models are around ⅝” to ¾” but newer models can be ½” or less. While automatic bar screens are capable of removing more debris with less manual labor, there is still a skill set required to properly operate this machinery. Operators will check the equipment multiple times during a day. Checks can include monitoring the rake arm and motor for proper function, removing compacted debris to a larger disposal truck, checking motor amps are within range, and calibrating level sensors.
Comminution and Barminution
An alternative to screen these large debris items is to shred or grind them. A comminutor is a device that sits inside the channel where wastewater is flowing into the treatment facility. The comminutor will grind the large debris items, turning them into smaller items. Communitors are designed to produce a solid size of a certain diameter. By breaking up the debris into smaller diameters, the downstream pumps and equipment will not be as impacted. It’s important to note that comminutors do not remove the debris from the flow of wastewater. The debris is just broken down into more manageable sizes. The debris will still need to be removed in the subsequent treatment process.
Barminutors combine a bar screen with a comminutor. There is a bar screen set inside of a channel where the wastewater flows. The debris is trapped behind the bars and then a device travels up and down the bars which breaks up the large solids into smaller pieces. The ground-up solids stay within the flow of wastewater and move onto the treatment process.
Grit Removal
Once the large items are managed, the next step in the treatment process is to remove the smaller inorganic solids, such as coffee grounds, eggshells, sand, silt, and gravel. These small diameter solids are collectively called grit . Grit must be removed because it will cause excessive wear on plant equipment such as the impeller of a pump. Also, this inorganic material can settle in the subsequent treatment process and take up valuable space in tanks which decreases plant efficiency and can inhibit further treatment. While most gritty materials are inorganic, large organic solids such as corn kernels and other food waste may also be removed.
There are several methods that can be used to remove grit but all of the methods rely on the fact the gritty material is relatively heavy. Compared to the organic solids, grit is much heavier and will settle faster. Grit can be removed by controlling the velocity of wastewater through a tank, adding air to the tank to aide in settling, or by using centrifugal force.
The basic concept of removing grit is to reduce the velocity of the wastewater flowing through a tank to less than 2 fps. Recall that in the collection system we want to achieve a velocity of 2 fps. The higher velocity in the collection system will keep all of the solids in suspension so they make their way to the treatment plant. But now that we are trying to remove these solids, we want to get the velocity between 0.7 and 1.4 fps. At this lower velocity, the heavy inorganic solids will settle to the bottom of the tank.
Detention time is a critical factor in designing the tank dimension for grit removal. The detention time needs to be long enough to allow the gritty material to settle to the bottom. The typical detention time of grit chambers is around 2 to 5 minutes. However, if the detention time is too great, then smaller organic solids will also settle out. These items are better dealt with in the next step of primary treatment.
Aerated grit chambers are set up in a similar manner except there will be air piped to diffusers at the bottom of the tank. The addition of air in the tank creates a rolling action of solids which helps keep the lighter organic solids in suspension while the heavier grit material is directed to the bottom of the tank. In aerated grit chambers, the amount of air sent to the chamber is a critical operating parameter. If too much air is supplied, then the grit material will stay in suspension and not be removed. If not enough air is supplied, then the lighter organic material can settle out.
Cyclone separators are another method of removing grit. These separators use centrifugal force to push the gritty solids to the edge of a circular chamber where they are then directed to the bottom of the tank. To create the centrifugal force, cyclone separators will require a higher velocity of wastewater moving through the unit. Typically a velocity of 2 fps to 3 fps will be sufficient.
Any of the grit removal methods described will result in an accumulation of material at the bottom of the unit. If these solids are not removed periodically, they will accumulate to the point where the unit will be ineffective. The tanks are designed to direct the gritty material toward the bottom of the tank which is sloped towards one end. This allows the settled grit to be removed from the tank for further treatment. The mixture of grit and wastewater is pumped and sent to a grit washer. Grit washers will wash out the organic material and send it back into the flow of wastewater as it moves onto the next treatment process. This allows only the inorganic abrasive solids to be collected for ultimate disposal. Once the gritty material is washed and collected it is sent to a landfill.