Wastewater treatment is an important process in modern societies to minimise our impact on the environment. It involves the removal of contaminants and converting wastewater into effluent to be returned to the water cycle.
The treatment of wastewater creates a byproduct called sludge, which must also be treated and disposed of responsibly. Sludge is essentially a combination of organic and inorganic materials, chemicals and pathogens. It has a semisolid, slurry consistency and is typically brown in colour.
The goal of sludge treatment is to reduce its volume and stabilise the organic materials so it does not have a foul odour and is not a health hazard.
What Are the Types of Sludge?
Sludge is often categorised as primary sludge and secondary sludge. The difference between the two lies in how they are created.
1) Primary Sludge
Primary sludge is generated from chemical precipitation, sedimentation and primary processes. Primary processes refer to treatment methods that perform the initial filtration of wastewater such as screening, comminutor and the grit chamber.
2) Secondary Sludge
Secondary sludge is generated from biological treatments. Biological treatments are when wastewater is mixed with microbial cells which are specifically used to “eat” the waste in the water. The single-celled organisms are put into aerated water which helps them to multiply and cleanse the water. The resulting sludge from this process is defined as secondary sludge.
For treatment and disposal, primary and secondary sludges are typically mixed together for convenience.
How is Sludge Treated and Processed?
There are several steps in the sludge treatment process. Here is a step-by-step overview to help you get a better understanding of the entire process:
1) Sludge Thickening
Sludge in its default semisolid slurry state is extremely difficult to handle. To make treatment and processing easier, sludge is typically thickened in a dissolved-air flotation tank or gravity thickener.
A dissolved-air flotation tank thickens the sludge by using air bubbles to carry the solids to the surface, forming a layer of thickened sludge on the top.
A gravity thickener works in reverse, where the sludge is allowed to settle to the base of the tank. The thickened sludge is then extracted from the bottom of the tank, leaving the top diluted.
Both methods reduce the overall volume of the sludge and make it easier to handle.
2) Sludge Digestion
After the sludge is thickened and all the solids are amassed, it is ready for digestion. Sludge digestion is a biological process that decomposes the organic solids in the sludge into a stable substance. This biological process called anaerobic digestion destroys pathogens, making sludge safer to handle, less offensive, easier to dewater and reduces the total mass of solids.
Sludge digestion happens in two-stages. In the first stage, the sludge is heated and mixed in a close tank for several days. Acid-forming bacteria hydrolyzes the large molecules of proteins and lipids present in the sludge, breaking them down into small water-soluble molecules before fermenting them into various fatty acids.
The second stage continues when the sludge is sent to a second tank. Here, other types of bacteria break down the remaining organic compounds within the sludge to produce a mix of carbon dioxide and methane. Depending on the quantity of methane released, some of the gas is collected and fed back to power the digestion tank – further improving sustainability.
By the end of digestion, the sludge looks like potting soil and no longer smells foul.
3) Sludge Dewatering
After going through digestion, sludge can be handled as a solid material. However, at this point, the mass still contains a high amount of moisture, as much as 70 percent in some cases. The next step, dewatering, aims to remove this moisture and dry the sludge to further reduce its weight.
The digested sludge is spread out in an open area made out of sand and allowed to remain until dry. These areas are called sludge-drying beds. The sludge dries through evaporation and sometimes drainage is installed under the sand to pump the water as well. This process is natural but time-consuming, requires lots of empty land and subject to environmental weather conditions.
In urban, populated cities where space is a concern, mechanical systems such as the rotary drum vacuum filter, belt filter press and centrifuge are used to dewater the sludge. They work by consistently spinning the sludge at high speeds until liquid (water) is separated from the solids. Sometimes, chemicals are added to the sludge to coagulate the solids and make separation easier.
4) Sludge Disposal
After sludge has been digested and dewatered, it is finally able to be disposed of safely. The disposal method chosen is dependent on the available land and toxicity level of the sludge.
There are two main methods of disposal: landfills or incineration.
In landfills, the dewatered sludge is buried underground in a sanitary landfill. Depending on the sludge’s chemical composition, it can also be used as a soil conditioner or fertiliser. However, it is typically not used on farmland that grow crops for human consumption, due to the possibility of the sludge containing toxic industrial chemicals.
For urban cities, sludge is typically disposed of through incineration. Incineration evaporates all remaining moisture and turns the sludge into inert ash. The ash still needs to be disposed of, but the burning process greatly reduces the mass of organic solids within the sludge into an amount that makes more economical sense to dispose. However, air pollution is a legitimate concern when it comes to incineration. To prevent this, many incineration plants utilise air-cleaning devices like scrubbers and filters before the smoke is released into the atmosphere.
Sludge treatment is an undeniably important part of treating wastewater and ensuring our activities as a society are not a net negative on the environment.
At Advance Technic, we’re sludge treatment specialists focused on providing sustainable cleaning and waste management solutions for industries like refineries, petroleum, petrochemical and speciality chemicals.
Our team combines professional equipment with well-trained personnel and efficient methods to provide ideal services for industrial zones. Find out more about our services and what we can do for your company at www.advancetechnic.sg/our-services/.