Digestate treatment
In certain European countries, especially The Netherlands, Belgium and the French region Brittany, livestock production is so intensive that a surplus of manure cause problems. As a consequence in some regions farmers demand treatment technologies for manure which eventually enables them to export surplus manure fractions. Anaerobic digestion is known to remove sticky organic compounds from manure, which often makes for example separation of manure a lot easier. For that reason there is a special interest in the combination of micro scale biogas plants and digestate treatment.
As manure consist mainly of water, the transport of manure to places which have a shortage of minerals is very expensive. To get rid of the surplus of manure, different treatment technologies are used to concentrate the minerals in the manure. For farm-scale technologies at land-based companies, the main aim is to compensate the surplus of manure, rather than all the manure, as they can spread the remaining manure on their own land.
The main characteristics of digestate treatment technologies are discussed here. Please click on the different treatment technologies for more information.
Separation – Manure is separated in a thick and thin fraction
Quite popular is the technique to process manure/digestate by the use of a mechanical separation device, which results in a thick (30% dry matter) and a liquid fraction.
If needed, the thick fraction can be processed further into compost, or dried; the liquid fraction can be purified further by first removing the last bits of dry matter (by e.g. ultrafiltration or a dissolved air flotation unit) and then removing a large part of the minerals (by e.g. nitrogen stripping, struvite production or reversed osmosis). With the resulting end products, specific combinations of artificial fertilizer replacers can be created.
Screw-press separators
Screw-press separators are often used in intensive livestock housing with large manure volumes. They are also often installed in bigger biogas plants for recirculation purpose, to reduce digestate volume, or as first stage of further digestate processing. The dewatering process occurs through mechanical pressing of the digestate by a screw conveyor to an abutment. The released liquid is through a strainer with a column width of 0.1 – 1 mm discharged to the outside. The processed substrate has a dry matter content of about 4-15% and reached an average dry matter content from 18 to 35%. Because of the simple method, the dry matter content of the liquid phase is with about 3-4% relatively high.
The main components such as screw press and strainer are often made of stainless steel. The housing is made of cast iron, stainless steel or cast stainless steel.
The installed electrical capacity ranging from 3 to 11 kW. Main components are Separation unit, stand device and process control.
If the separator is not needed fulltime, a mobile separation (rental) could be more economical.
Decanter
Decanters can achieve more separation efficiency than screw presses. The separation mechanisms are different; decanters are based on the centrifugation of the digestate, while in the screw press the digestate is compressed using a filter of a particular mesh-size and a screw. Decanters are not often used in small-scale biogas plants due to high operating costs.
Composting - A batch of thick fraction is composted
Composting is a biological process in which organic matter is aerobically (in the presence of oxygen), converted into humus-like compounds by micro-organisms. During this process heat, water, CO2 and odour compounds are released. Due to the evaporation of water and the decomposition of organic matter, the dry matter content increases and the volume decreases (Melse, et al. 2004). Composting installations for the composting of manure are offered by various parties. Manure is firstly separated into a thin and a thick fraction. The solid fraction is composted in a drum. The thin fraction is stored. Composting is an aerobe process, the material is therefore aerated, either with air bubbles or by mechanically turning the material. If there is insufficient structure or carbon present in the material, additional material such as straw can be added to get the process started. During the composting process heat is released, if the process is properly controlled, temperatures can reach over 70 ° C and the process can be valorised.
Drying - Digestate is dried and valorised so it can be exported
Nitrogen stripping – Ammonium-sulphate production
Nitrogen stripping is widely used in industry. Different techniques are available, but the principles are the same: In the stripper, ammonia is driven from the medium by optimizing pH and temperature, to a gaseous phase (air). This air is then washed in a scrubber with sulphuric acid.
Binding the ammonia with sulphuric acid in a scrubber is widely used in (for example German & Dutch) agriculture, for example in piggeries, where acidic washers prevent the exhaust of ammonia from stables. The end product is a (as chemical fertilizer recognised) nitrogen-rich fertilizer. If the temperature of the stripper is high enough, the stripping process can be valorised.
Struvite production – production of a nitrogen and phosphate rich fertilizer
Struvite production is based on phosphate precipitation, a widely used technique in water treatment facilities to extract nitrogen and phosphate from sewage water. The phosphate is bound to magnesium and nitrogen to form struvite (Mg (NH 4) PO 4 • 6 (H2O). This struvite is a mineral / crystal. Because the crystal is formed in the digestate it may still contains some organic particles, it can therefore not be valorised unless the struvite is pasteurised.
The process of phosphate precipitation is as follows:
The manure is pumped into a vessel together with caustic soda and magnesium chloride. In this vessel, the orthophosphate reacts (the non-bound phosphate in the digestate) together with the nitrogen, with the magnesium, and forms a crystal. This crystal grows when it comes into contact with more of phosphate, nitrogen, and magnesium. When it is large enough, the crystal is separated from the manure. The crystals are washed so that a “clean” struvite remains after this separation. When struvite gets a heat treatment (pasteurization) in can be exported.
By struvite production, the minerals are concentrated and thereby reduce the number of transport movements to get rid of the surplus of manure.
EU regulation (EG 1069/2009 for animal by-products) prohibits the export and transport of non-pasteurised manure. A pasteurisation step should therefore be included, or the used technologies should be valorised (the end product is recognised as pasteurised manure). For more information about EG 1969/2009 click here. The following treatment technologies can be laid out conform to the EG requirements:
- Composting – A batch of thick fraction is composted (can be valorised depending on the process).
- Drying – Digestate is dried and valorised so it can be exported.
- Nitrogen stripping – (end product is valorised and recognised as artificial fertilizer)