- Compare and contrast the different methods of biosolids dewatering
- Understand the limits of different biosolids dewatering methods
- Compare the energy and labor requirements of different biosolids dewatering methods
Sludge dewatering is exactly what it sounds like. At this point, the sludge has been stabilized by reducing the amount of volatile organic material. It’s almost ready for disposal which usually means it will need to be transported somewhere. This can be very expensive because the sludge is still mostly water. Dewatering removes a lot of the water and increases the percent of solids. This will make transporting the dewatered biosolids much more cost-effective.
The simplest and cheapest way to dewater sludge is by drying beds. The sludge is sent to the drying beds where the sun heats it up and evaporates the water. This process can take several weeks or even months to achieve the desired percent solids. Also, the process is dependent on the weather. In colder months it can take even longer. There are some adaptations to increase the process. Some drying beds will be slightly sloped with sand in the middle. There is an underdrain system beneath the sand. With sand beds, the water is being evaporated by the sun but is also being directed towards the sand and filtered through to the underdrain system where it is then sent back to the headworks of the treatment plant. There are also vacuum assisted drying beds. Similar to sand drying beds but instead of relying on gravity for the water to drain a vacuum is created to force the water out. Drying beds are efficient and cost-effective for smaller systems and where land is available.
A belt filter press consists of two long filters. The sludge will be conveyed in between these two filters and then sent through progressively higher areas of pressure where water is squeezed through the filter and the solids are left behind. The first part of the belt filter press is the gravity zone. Here the digested sludge is mixed with polymer and begins to coagulate the solids. The solids are then conveyed and sandwiched between the other filter. There is then a low-pressure section where water is forced between the filters and removed. The pressure then gradually increases as the filters are rolled through the belt press. This gradual increase allows more and more water to be removed. At the end of the belt press, the two filters separate and the biosolids are scraped off and sent to a conveyor belt. Belt presses can achieve around 13% to 18% solids. Oftentimes belt presses are combined with drying beds to further increase the percent solids.
There are a couple of different types of filter presses. The most common is the plate and frame filter press. The unit consists of a series of filter plates. The plates are forced together with a hydraulic press and sludge is conveyed in between each of the plates. As the sludge is pumped into the filters the solids are trapped by the filter and the water passes through and is collected in a drain system. As more and more sludge is pumped into the filters the pressure will begin to increase. This extra pressure will cause even more water to be forced through the filters and out of the sludge. When the pressure reaches its maximum the operator will stop feeding sludge to the filters. The plates are then released from the hydraulic press and separated. As the plates separate, the dried biosolids will fall off the filter plates and drop below to either a conveyor belt or oftentimes to the bed of a dump truck. The biosolids will then be sent off for ultimate disposal. A well-operated plate and frame press can achieve a solids concentration of 40% to 50%.
Another type of filter press is vacuum filtration. This consists of a circular drum with a filter material on the outside. The drum is submerged into a trough filled with the digested sludge. The drum slowly rotates while a vacuum is being created inside the drum. The vacuum pulls the water out of the trough. The solids are stuck on the outside of the filter while water is able to filter through and be sent to the drain system. By the time the drum does a full circle, the percent solids has increased significantly and the biosolids are scraped off to a conveyor system where they are sent to a dump truck.
Waste Stream Recycling
The water being removed from the dewatering process needs to be sent back to the liquid portion of the treatment plant to be further treated. However, this water is often very high in ammonia and can overwhelm the bacteria in the activated sludge process. Some treatment plants will slowly pump this water back to the treatment plant so there isn’t a large slug of ammonia going through the system. Others will only pump the water back during the night time hours when ammonia coming into the treatment plant is often lower. Some newer technologies will actually treat the water prior to being sent back to the headworks.
Now that the sludge from the primary sedimentation and secondary clarifiers have been stabilized and dewatered we can call it biosolids. The biosolids need to either be disposed of or reused. Biosolids are classified into two separate types by the EPA. Class B biosolids have been treated by the processes discussed in this chapter but can contain high levels of pathogenic organisms. Therefore, Class B biosolids have greater restrictions on land application and crop harvesting. Class A biosolids typically undergo a combination of the treatment processes discussed in this chapter to achieve lower levels of pathogens. Class A biosolids that meet EPA regulations can legally be resold as fertilizers.