January 19th, 2009 by Will Kirkman
Anaerobic digestion (AD) is the biological degradation of organic material in the absence of oxygen.
The Bioplex AD system treats high solid content organic wastes and, if required, liquid organic wastes. The system is operated as a batch system and in two phases this process is the patented Bioplex process: -
The first or solid phase uses a digester vessel (called Portagester e.g. PG1, PG2, PG3, PG4, which vary in capacity.)
This portable digester acts as anaerobic filter/leachate bed reactors pasteurising and stabilising the feedstock. The vessel is covered and positioned at the digestion site; liquor from the liquid phase (Stage 2 digester) is circulated through the contents during the digestion period, washing out organic fraction – this stage of the process requires heat. This solids phase typically has a 3-day (or less) retention time, during which the stabilisation known as acid phase hydrolysis occurs. At the end of the process period, the liquid is screened, drained out and stored in the second stage digester where a process known as methogenesis takes place. The methane generated from this process is used for energy either to heat the first stage or put through a generator to produce electricity. The solids left from stage one are relatively odourless and can be safely spread onto farmland or composted, typically 21 days, which breaks it down more effectively. The composted material is an excellent natural fertilizer and growing medium.
The process produces a treated solid material that can be composted to produce a natural fibrous fertiliser or biofertiliser from the first stage and biogas from the second stage.
Each tonne of feedstock may produce 25 to 120 kgs of biogas (depending on what the feedstock is) and if used for energy can yield 80 to 400 kgs of carbon dioxide. When the other digester output, the biofertiliser is used in agriculture or horticulture, the mass of increase in plant growth exceeds the carbon dioxide from the energy production. The whole process can therefore absorb atmospheric carbon dioxide.
Depending on what is to be treated, each tonne of feedstock may produce 25 to 120 kilograms of biogas at 60 to 75% methane (CH4) depending on what is treated.
Generally any organic or putrescible waste material that is non-toxic can be processed by this system including raw food; cooked food; food processing waste; food waste from retail and catering; meat processing by-products; farm and zoo animal manures; horse stable manure; industrial wastes e.g. paper pulp, fats, oils, greases; sewage and industrial sludge; household and trade refuse – municipal solid waste (MSW) organic fraction (OF).
The first stage of the process takes two to three days after which the digested solids are ready for composting or other uses. The composting or aeration of the digested solids may take 10 to 30 days subject to end use, regulation and method. The whole process to a usable compost can take as little as 28 days.
The screened liquid from the first stage is contained in the second stage digester where the liquid produces biogas (methane), this liquid is used to “wash” the first stage and is permanently stored in the stage two digester.
The AD process reduces the volume of waste by 25 to 90% depending on the nature of the feedstock.
The running costs are as follows:
a. Labour costs of operation, for example one to two hours for digester systems treating up to 5,000 tonnes per annum (tpa).
b. Maintenance is subject to any service contract and use of the apparatus.
c. Services such as electricity, water and drainage should be considered.
After treatment in the first stage digester, the solids have been drained of surplus fluid, tipped out and composted. The composting or aeration ensures that the biofertiliser is stable and does not draw oxygen from the soil.
The combined digestion and composting process controls odours, parasites, pathogens and weed seeds.
The anaerobic process alters the way in which the nitrogen and other nutrients are held within the biofertiliser, keeping a large proportion as slow release elements. In addition unlike synthetic fertilisers the biofertiliser also contains humus and beneficial microbes. With nutrients for plant growth, what goes into the digester generally comes out in the biofertiliser.
Nutrients are in a stable slow release form and are “locked in” with the fibre. A heap of biofertiliser does not scorch or kill plants like raw manure or slurry, nor do the nutrients leach out as readily.
In many cases there is no surplus liquid from the Bioplex Technologies Ltd process due to the high solids content feedstock such as farm animal and horse stable manure (including bedding). If the feedstock has low solids content such as farm animal slurry, then there will be treated liquid biofertiliser can be used as a liquid plant feed and in some cases added into a compost operation. The latter accelerates the composting and adds to nutrients in the end product.
If the feedstock fed into the digester is rated as organic then the biofertiliser produced can be used on an organic farm. In some cases solid digestate may be more easily classed as organic than liquids. You are strongly advised to check with your organic growing organisation to make sure the biofertiliser is suitable for use.
Systems treating 300 to 1,500 tonnes per annum use a 13 amp 230 volt I phase electricity supply for pumps, lights, controls, kettle and the radio at a rate of 2 to 10 kWh per day. Smaller systems may require energy for digester heating as heat losses may exceed biogas to heat. Larger systems may require 3 phase supply for the operation.
Systems treating more than 250 tonnes per annum may be energy self sufficient and ones above 5,000 tonnes per annum could provide surplus energy for sale.
Much depends on the quantity and type of material treated, whether a boiler or combined heat and power (CHP) generator is used, the operation of the plant and cost efficiency of exporting energy.
Bioplex prefers to store the minimum of biogas (methane), on site, used for the operation due to the cost of gas storage vessels and safety implications.
Gas is stored in flexible membrane “bags” or floating roof gasholders (gasometers) or other means.
Biogas/methane does not liquefy under pressure at room temperature, like propane or butane so other methods such as absorption, chilling and high-pressure storage have been developed for uses such as transport.
The odours created by the process are contained within the digestion vessels and piping. Most odours occur when loading odorous materials and spillages on the site. A well-run digester does not produce odours to the atmosphere; the treated output material smells of freshly dug earth. It can be a good neighbour.
The digestion process itself is silent; noises are produced by pumps and other process equipment. An insulated plant room will ensure minimal noise levels.
The Bioplex Process uses a batch system of operation, so it doesn’t operated constantly. The first stage digester can be loaded and unloaded relatively quickly and should take minutes each day. A well-designed plant can work for most of the day unattended. A daily walk around the plant is also advised.
Operation of the system is relatively simple based on batch filling and unloading. Unloading can involve tipping out. The system has been designed to prevent and reduce pump and pipe blockages, silting up and operation downtime.