The anaerobic digestion is a sustainable method to convert organic waste materials into renewable energy and recycled nutrients such as nitrogen and phosphorus.
The biogas plant is a chain of separate unit processes that must work in series without any interruptions.
The reception of the waste materials is the first operational part of the whole biogas plant. In this process the materials (both liquid and solid) are poured into a tank that takes materials into its depths and supplies the biogas plant following processes without critical interruptions.
Odour is very-well recognized local environmental issue in all waste treatment facilities. Watrec approach to solve this issue starts already from the reception unit, which has closed hatch and efficient, constant odour removal flow that is directed to Watrec odour removal systems.
In the reception tank the materials are mixed and unwanted obstacles are left in the tank.
The odour treatment systems applied in Watrec biogas plants contains different units, such as bio-chemical scrubber and activated carbon filter. The system is low-cost in its operational terms and produces removal efficiency of more than 95 % from odour concentrations.
The materials that are delivered to the following unit processes are conducted through a crusher unit to meet the EC animal by-product legislation requirements.
The three-phase heat exchanger systems are one of the key-elements of the Watrec biogas plant. This operation regulates the temperature of the waste materials simultaneously to 70 ˚C and then back to mesophilic temperature range.
The system operates in three phases:
- In the first phase the inputflow is preheated by recirculated heat energy from outflow of the hygienisation units.
- On the second phase the sludge is heated up to 70ºC by hot water from the CHP unit (or other external heat source) in sludge – water heat exchanger.
- Then the sludge is cooled to 35-38ºC prior entering into a digester.
The continuous recovery of the heat due to counter-current flow design results as superior thermal energy use; only 20 to 30 % of the supplied heat produce in the biogas plant CHP is consumed for hygienization because of the three-phase heat exchangers.
Hygienization ensures the complete elimination of pathogenic bacteria and improves also the biogas production due to particle hydrolysis – this feature can have more than 20 % added total energy efficiency.
The heart of the biogas plant is it’s digestion process which can be divided into four separate steps: hydrolysis, acidification, acetogenesis and methanogenesis.
The important part of the digestion process outputs is the production of nutrient materials. Considering the mass balance of the whole biogas plant; approximately 5 to 10 % is directed out from the process as biogas flow. This leaves the remaining fraction, called a digestate, to owe more than 90 % of the volume of the materials that go through the treatment process.
Mechanical dewatering in Watrec biogas plants is used to produce precision fertilizers; fiber fraction and reject water fraction. Production of recycled nutrient materials in Watrec biogas plant enables us the possibility to call our solutions to be really a biorefinery.
In Watrec biorefinery systems either decanter centrifuge or screw press is applied for dewatering depending on the feedstock and required nutrient balance between fertilizer products. In the dewatering process two different fertilizer streams are produced, dewatered digestate (fiber fraction) and reject water. Fiber fraction typically has a dry solids concentration of 25-30 % and has 1:1 ratio between nitrogen and phosphorus. Reject water has notably higher volume, approximately 85 % of the total input to the plant, and owes 85 % of the ammonia in the products – together with low concentration of phosphorus.