Sludge issue: How does WWTP work?

Wastewater is an everyday element of any human activity. Every day, hundreds of litres of wastewater flow from our toilets, bathrooms and kitchens into the sewer system. Then it flows into wastewater treatment plants (WWTPs) where raw wastewater is biologically treated and sludge is formed. The sludge contains all the pollution that comes out of households, industrial and agricultural operations, along with the water. It is the sludge and its dewatering, transport, storage, and, most importantly, further use that represent an economic and environmental challenge for today's wastewater treatment plants.

WASTEWATER: BLACK, GREY AND INDUSTRIAL

Sewage is wastewater from households and sanitary facilities. Depending on pollution it is categorised into black and grey: black, heavily polluted wastewater comes from toilets; grey, less polluted, comes from kitchens, bathrooms and washing. Sewage water can contain up to 80% pollutants. These consist of a mixture of solids from excrements, urine, food and detergents.

Water from industrial and agricultural plants goes to wastewater treatment plants along with municipal wastewater from households. This includes waste water from breweries, wine producers, dairies and cheese factories, meat processing plants, as well as industrial laundries, galvanic or zinc plating facilities.

WASTEWATER TREATMENT PLANT

We can simply say that wherever water (that is not transparent) is a by-product of human activity, this water contains solid pollutants that we need to get rid of. This is where wastewater treatment plants, or "WWTPs", come in.

The water that flows into the wastewater treatment plants first of all undergoes mechanical pre-treatment on belt filters or drum filters. There, gravel, sand, grease and other solids are trapped. Then comes biological treatment. By adjusting the suitable conditions, such as the amount of oxygen, temperature, or water residence time, we achieve presence of „right“ bacteria, which then eat the solids dissolved in the water and excrete them as suspended solids. These form sludge. While water that meets the health limits for pollution content is drained into rivers, reservoirs or sewers, sludge is retained and accumulated in WWTPs. It has the consistency of thinner or thicker mud and all the physical, biological and chemical pollution remains in it.

AND WHAT ABOUT THE SLUDGE? DEWATER IT!

Sludge contains units of percent solids, i.e. impurities. This thick polluted wastewater cannot be discharged into the river and, because of its huge volume, is difficult to transport and dispose of. So how can we deal with the sludge? The best solution is to use dewatering to get it to a state where it is as easy to handle as possible. After dewatering in the dewatering screw presses, the originally thin sludge becomes a solid with a cohesive consistency that goes out on screw or belt conveyors. This dewatered sludge can be easily transported or stored due to its dramatically smaller volume.

FLOCKS ARE NOT JUST SNOW

However, dewatering itself would not be possible without the use of a flocculant. In WWTPs, polymers, substances with a structure similar to plastics, are most commonly used as flocculants. They act like magnets on a microscopic level. Due to the different electrical charge of the polymer chain and the surrounding impurities, the polymers bind the microscopic solids dissolved in the sludge as they pass through. This creates larger clusters of particles - flocks, half a centimetre to a centimetre in diameter.

These chemicals are dosed and mixed into the sludge at polymer units. In a manual polymer station, this is done manually by the operator, while automatic polymer preparation units can continuously mix the polymer themselves. Once the polymer has been added to the sludge, it is no longer a homogeneous sludge, but almost transparent water in which flocks float - clumps of solid particles. This is important for the dewatering presses: they let the clean water out, and the flocks are retained, displaced and squeezed. Even more water is squeezed out. We've got the sludge dewatered, so what is the next step?

WHERE TO USE DEWATERED SLUDGE?

What to do with the solids that come out of the dewatering machine? We have several possibilities:

• Drying. It's a pretty costly affair, both energy-intensive and financially. Compared to dewatering, it consumes up to 100 times more energy. The volume reduction is not as significant as with dewatering. Drying is therefore an option that is used only rarely. It's not really done in practice.

• Combustion. Currently a very promoted trend.

• Composting. The dewatered sludge goes to the composting plant where it is sanitised either by pressure or heat. This is a process that eradicates the bacteria and viruses still present in the sludge. It is then mixed with raw clay soil to produce high-quality, nutrient-rich sludge which can be spread on fields, for example. However, composting has recently been quitted. Dewatered sludge contains microplastics from toothpaste and shampoo, as well as those released from synthetic clothing during washing. Another problem with composting is that the processes involved are difficult to monitor by regulatory authorities.

• Pyrolysis. In pyrolysis, sludge is separated into three chemical components at high temperatures: residual water, organic carbon and oil. The oil, which contains residual microplastics and other substances, can be used as a source of energy for heating. However, today's pyrolysis technologies again require a lot of energy.

• Landfills. This rather unpopular option from an ecological point of view can be considered completely eradicated in today's EU. 

• Animal feed. Dewatered sludge is not always waste. However, it depends on where the sludge comes from and what kind of waste products it originally contained.

CUSTOMISED TO THE NEEDS OF WWTPs

Each wastewater treatment plant has specific requirements for sludge treatment. Whether it is mechanical pre-treatment, polymer dosing, sludge dewatering in presses, or final transport of the sludge... Over the years, we have developed top sludge processing machines and treatment equipment and accessories that are successfully used by wastewater treatment plants all around the world. We can find a customised solution even for you.

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