Reducing sludge in wastewater plants is one of the most expensive and challenging parts of wastewater treatment. Managing, transporting, and removing sludge requires advanced equipment, time, and lots of hard work. In this article, we take a closer look at different techniques for reducing sludge in wastewater plants (including physical and chemical ones) and provide you with practical tips for sludge reduction.
Why Is Reducing Sludge in Wastewater Plants Important?
Dewatering, drying, and transporting sludge are costly and challenging; therefore, minimizing these processes can lower operational and maintenance expenses to a great extent. Plus, sludge bulking applies pressure to the plants’ equipment, especially on the thickeners, digesters, and dewatering systems.
Reducing sludge can also improve the plant’s sustainability, free up its capacity, and lower greenhouse gas emissions. Some techniques even convert sludge into clean energy or useful byproducts (e.g., biogas), which reduces the carbon footprint.
5 Techniques of Sludge Reduction in Wastewater
Here are the top 5 techniques for reducing sludge in wastewater plants.
1. Adjusting the Treatment Process.
- Use anaerobic or hybrid systems.
Aerobic systems (with oxygen) make more sludge because bacteria grow fast. Adding an anaerobic stage (without oxygen) changes more organic matter into gas instead of biomass. It means there will be less sludge in the treatment basins. Needless to say, a mix of both systems can balance the sludge production.
- Keep sludge longer (increase SRT).
Letting sludge stay longer in the system allows old bacteria to die off naturally and slow-growing microbes like nitrifiers to grow. This simple technique (increasing sludge retention time) reduces the sludge volume, but it can densify the sludge and make the dewatering procedure even harder.
- Avoid overloading the system.
Too much organic load causes fast bacterial growth and more sludge. Use flow control or equalization tanks to smooth out high peaks.
- Try controlled chemical or biological stress.
Some plants use mild chemicals or special conditions to slow down bacterial growth and reduce sludge. This needs careful control.
2. Making the Sludge Settle Better.
When the sludge settles, it is easier to manage it. So, the following 3 techniques are usually used to help it settle at the bottom of the tank.
- Control filamentous bacteria.
If the number of filamentous bacteria exceeds, it causes problems like sludge foaming and bulking. In such a situation, the sludge becomes so fluffy and large that it can’t settle well. Although some filamentous bacteria produce helpful bacteria, the engineers keep their oxygen, nutrients, and loading balanced to prevent sludge bulking.
- Optimize return activated sludge (RAS).
RAS refers to the amount of sludge that comes from the secondary clarifier to the aeration tank. Too much or too little of such sludge can affect floc structure and settling.
- Use the right amount of polymer.
In most treatment plants, certain chemicals –like polymer- are added to sludge to make the solid particles stick together. The right type and dose of polymer can make sludge thicker and easier to dewater.
3. Improving Thickening and Dewatering Process.
To reduce the sludge volume in wastewater plants, the engineers remove the water from it.
- Mechanical pressing
The operational staff mainly use belt, screw, or plate presses to dewater the sludge. These heavy machines are suitable for thickening a large volume of sludge.
- Centrifugation
Machines like decanter centrifuges, solid bowl centrifuges, and filter cloths are energy-efficient and remove even more water from the sludge than pressing machines.
- Drying
Dry sludge is lighter, so it is easier and cheaper to move or get rid of it. There are two eco-friendly ways to dry the sludge:
- a) drying beds: the sludge is moved to a permeable bed where water drains naturally. This is a simple and low-cost method, but it is slow and requires a lot of space.
- b) Geotextile dewatering tubes: the sludge is stored in large bags where the water drains over time, without human or machinery intervention.
4. Using Digestion and Pretreatment Methods.
Anaerobic digesters break down the sludge particles and turn organic matter into biogas. However, some sludge has a hard cellular structure that prevents digestion. That’s where the pretreatment methods are used. Pretreatment breaks down these hard cells, making it easier for bacteria to ingest. Common pretreatment methods include:
- Thermochemical hydrolysis
This method breaks down sludge cells and makes them biodegradable by using heat and chemicals.
- Ultrasonication
The engineers sometimes break down the sludge flocs by using high-frequency sound waves. They use ultrasound after adding chemicals or adjusting the pH of the sludge to increase its effectiveness.
- Other pretreatment methods
Other strategies, such as grinding, chemical oxidants, and enzymatic hydrolysis, are also used to break down the sludge before digestion.
5. Using Waste Lagoons.
Lagoons need space and constant monitoring, but they’re a low-cost option for reducing sludge in wastewater plants. In the lagoons, solid pollutants settle at the bottom to create a sludge layer (resembling a sludge blanket). After settling, liquid is separated from solids, and dewatering equipment like centrifuges reduces the sludge volume by up to 30%.
4 Tips for Reducing Sludge in Wastewater Plants
It takes a lot of time and effort to reduce sludge in wastewater plants, but engineers can accomplish it using these four tips.
- Some methods, like thermal drying or ultrasonification, use a lot of energy. Therefore, they need to check if it is worth the extra power or chemicals.
- Reducing too much sludge too quickly can damage the treatment process. The engineers should constantly monitor the system and record the changes.
- When the sludge volume is reduced, the remaining sludge may have more concentrated pollutants. So, they should make sure it is still safe for the environment.
- Some areas have strict sludge disposal laws or offer rewards for nutrient recovery. Local rules can affect which method makes the most sense.
The Last Words
Reducing sludge in wastewater plants is all about finding the right balance between costs, energy use, and time spent. There’s no single method that works “the best” for every plant. Actually, the best results come from merging the biological processes, thickening and dewatering techniques, and digestion or pretreatment methods.
AIMEQUIP is an industrial machinery provider that focuses on reducing wastewater plants’ costs, protecting the environment, and upgrading wastewater treatment facilities. You can reach out via our website or our LinkedIn page.
FAQs
- Can sludge be reduced to zero?
No. The sludge will always form at the bottom of the tank. Removing it entirely is almost impossible because it needs advanced technologies, money, and energy. - Is anaerobic digestion always better?
Not always. It is a proper solution for reducing sludge, but it depends on the plant size, wastewater type, energy use, and cost. Some plants still need aerobic treatment, too. - Does reducing sludge affect water quality?
It might if it is reduced too much. That’s why the engineers always monitor and control the process to keep the sludge level balanced. - How much sludge reduction is possible?
It varies by plant and method. Many plants reduce sludge by 20–50%.