Sludge and its processing

Sludge treatment is a significant challenge for cities, municipalities, and companies that manage wastewater treatment plants. It is a complex problem that involves technical, environmental, and economic aspects. In addition, sludge treatment is affected by legislative regulations and sustainability requirements. One possible solution is sludge dewatering and drying. This not only simplifies their subsequent disposal but also opens up new possibilities for use - for example, in the energy or agricultural sectors.

In this article, we will focus on an overview of currently available technologies for sludge drying and present a potential innovative alternative that may be an interesting novelty not only for the largest WWTP.

What are the disposal options for dewatered sludge?

The amount of sludge is slowly increasing, and the availability of downstream sludge drying influences the way in which dewatered solids are treated. (Development of sludge production and disposal in Czech republic, provides Czech Statistical Office)

• Composting: Affordable, but with reservations

Composting is the most common way of dewatered sludge disposal. It is a relatively affordable solution where sludge is mixed with other ingredients and gradually undergoes biochemical processes in which micro-organisms break down organic matter. The result is a stabilised and hygienically safe compost that can be used in agriculture or landscape restoration. However, several problems remain - the content of heavy metals, hormones, pharmaceuticals or pathogens can limit the use of the resulting compost, and the moisture and temperature of the sludge can slow down the composting process.

• Land reclamation (or agriculture use) : demanding conditions

Another option is to apply sludge to the soil or use it in land reclamation. This approach, however, faces strict requirements on sludge composition that are not always easy to meet. Failure to comply with these conditions can lead to soil & ground water contamination.

• Landfilling: unsustainable and costly

Landfilling is the most costly option, and one that pushes the problem into the future rather than solving it. It is therefore also the least attractive option.

• Pyrolysis: Ecological but expensive

Pyrolysis is an advanced method that is the most attractive option regarding environmental impact. The organic matter in the sludge is decomposed into pyrolysis gas, bio-oil, and biochar at high temperatures (300-900 °C) in an oxygen-poor environment. Each product has specific applications, for example, in the energy or chemical industries. 

Dewatering and drying - one without the other doesn't work

    1. Sludge dewatering

Mechanical dewatering is the first step in sludge treatment and a key prerequisite for further sludge treatment. This results in sludge dry content up to 25% (sometimes even more). For smaller treatment plants (< 2 500 EO), mechanical dewatering is often the optimal investment in case of sludge handling. However, it is economically and technologically advantageous for larger plants (> 10 000 EO) to extend dewatering with downstream technologies that allow further sludge treatment.

    2. Sludge drying

Sludge drying increases the dry solids content after mechanical dewatering from 10-25% DS to 40-90% DS, ensures hygienization of the sludge, and improves physical properties (e.g., burnability or storability) while reducing volume. This allows sludge to be used more efficiently in agriculture, energy, construction, and other sectors.

Sludge drying - what technologies exist?

• Thermal dryers

Thermal dryers operate on the principle of evaporation of water by the heat generated directly in the technology (or close to it) by heating elements/gas combustion or by an indirect heat source (e.g., waste heat from power plants, heating plants, cogeneration units, refrigeration,...) or a combination of these sources, which often increases drying efficiency and reduces operating costs.

• Non-thermal dryers

Non-thermal technology is almost synonymous with solar dryers (and + recently, electroosmotic dewatering). Solar dryers work on the principle of a greenhouse, where the sludge is spread over a concrete floor, and the evaporation of the water occurs by the action of solar energy together with a controlled ventilation system. 

Comparison of consumption, dry matter output, and disadvantages of different technologies

What is the ideal sludge treatment solution? Electroosmotic dewatering MP-EDW

The principle of this technology is a combination of electrophoresis (an electric field causes the separation of solids from water) and electroosmosis (water is pushed into the separation belt/membrane by the electrostatic Coulumb force generated by the electric field).

At the plant's inlet, the sludge is spread on a conveyor belt (cathode), which follows the circumference of the central drum (anode) in terms of trajectory. A voltage is applied across the electrodes (whereby the cells in which the water is bound are disturbed), which causes a current to pass through so that the solids are attracted to the drum and the water is pushed towards the perforated belt. The whole dewatering process is thus free of dust, odor, and gases.

What are the main advantages of this technology?

• The guaranteed energy consumption 0.25-0.45 kWh/l per liter of removed water.
• Low initial investment and operating costs.
• Minimal space requirements (10 m²).
• Automatic operation.
• Material processing speed of 3 minutes.

MP-EDW catalogue for download

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