Valorising sludge and slurry

Municipal waste treatment plants, among other processes, generate sludge and slurry-type waste that requires treatment. It is necessary to apply different technologies in line with the characteristics and components of the waste to be treated.

Waste can be processed and valorised using waste-to-product methods, thereby generating fuels or products that can be used as fertiliser for agricultural soil.

 

Sludge treatment technologies available nowadays

There are several ways of treating sludge/slurry – using different technologies, in line with the amount of moisture it contains: options include drying and/or pyrolysation to generate char, anaerobic digestion (for biomethanisation) and aerobic digestion (ATAD).

  • Drying: For the drying process, treatment takes place in pyrolytic kilns. Because these kilns lack oxygen, the sludge is converted into fixed carbon (char) that can subsequently be used as agricultural soil.

 

  • Pyrolysation: Pyrolysis is the chemical decomposition of organic compounds in the absence of oxygen. The process usually takes place at temperatures between 450 °C – 600 °C at negative pressure, and aims to break the long hydrocarbon chains into shorter ones. During this process, syngas (or synthesis gas) is generated by the volatile compounds that would normally be sent to a combustion chamber for oxidation. The process also generates a solid, carbon-rich waste material known as char. Pyrolysis is the first stage in the gasification phase for combustion. It can be carried out in two types of kiln: rotary and static. Rotary kilns are recommended for large volumes of waste (1-5 t/h) and static kilns for smaller volumes.

They are used to treat plastic polymers, which generate a syngas upon decomposition that can be subsequently condensed, distilled and used as a fuel.

Charring of wood to produce fixed carbon (char), which is used to improve soil productivity for agriculture. For this particular application, it is important that the wood and biomass do not contain heavy metals, as these are hazardous to the soil.

Sewage sludge: this must be dried beforehand, and can also be pyrolysed. Similarly, the aim is to generate fixed carbon for use in agricultural soil.

 

  • Biomethamisation: Sludge can also be digested anaerobically. This process is based on the production of microorganisms that break down the matter and generate biogas (biomethanisation).

Biomethanisation (or anaerobic digestion) is a biological process whereby, in the absence of oxygen and over the course of several stages involving a heterogeneous population of microorganisms, the most degradable part of the organic matter is converted into biogas, which is a mixture of gases comprised mostly of methane and carbon dioxide, along with smaller amounts of other gases (water vapour, CO, N2, H2, H2S, and others).

The mixture formed by these gases is known as biogas, which has a high calorific value that enables it to be used as fuel.

Anaerobic digestion can be wet, in which less than 20 % of the matter is dry; or dry, in which 20-40 % of the matter is dry.

The biggest advantage of this process is that, in addition to treating urban/industrial wastewater, it also generates biogas using microorganisms. Owing to its high calorific value, this biogas can be used in a variety of different applications, such as fuel to power boilers and cogeneration systems.

 

  • ATAD: Another option is ATAD technology, which comprises aerobic digestion (with oxygen) at a specific temperature for a particular amount of time (55 °C and 24 hours, approximately). This method renders the sludge free from pathogens, so it can then be used as agricultural soil.

ATAD stands for Autoheated Thermophilic Aerobic Digestion. This technology uses the nutrients and metabolic heat contained in sludge to generate high temperatures in a digester.

The process takes place in a closed tank, which is agitated and has air injected into it in order to aerate the sludge.

Its main application is in the treatment of wastewater in small towns and cities (i.e. those with 5,000-50,000 inhabitants).

It can be a good option for small and medium-sized plants, where the amount of sludge produced is insufficient for the implementation of energy recovery processes (anaerobic digestion)

During the process, the temperature is increased to 55ºC and remains at that level for 24 hours in order to sanitise the sludge and eliminate pathogens. The result is less concentrated sludge, which is then dehydrated and used as agricultural fertiliser.

 

At Tecam we offer a comprehensive range of services and a close, collaborative, continuous working relationship to address your emissions- and waste-related problems. We will advise you in your search for solutions, provide you with the necessary technology to implement your chosen solution, and accompany you in your daily operations in our capacity as your technology partner.

 

Contact us today if you have needs in relation to sludge/slurry waste treatment.

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