Poultry litter to biogas: adding more value to farm waste

The benefits of using chicken litter as fertiliser are well known. As the poultry industry grows, however, other uses for it must be found. Dr Idan Chiyanzu and Primrose Magama of the Agricultural Research Council’s Institute for Agricultural Engineering explain how energy can be generated from litter.

Poultry litter to biogas: adding more value to farm waste
Poultry litter, which is often used as an organic fertiliser, usually consists of bedding material, feathers, manure and wasted feed. Photo: FW Archive
- Advertisement -

Smaller poultry businesses, comprising mainly broiler and egg producers and suppliers of day-old chicks, are helping address the need for job creation. But this growing industry has a problem: how to manage the environmental impact of ever-greater volumes of poultry litter.

The premise of our research is that the industry needs to shift away from direct use of litter as a fertiliser, due to the high quantities of nitrogen, phosphorus and potassium
that accumulate in agricultural soil.

Instead, individual farmers and entities, such as co-operatives, have a responsibility to utilise the litter produced by their businesses for deploying renewable energy-generation technologies for a more sustainable and efficient economy.

- Advertisement -

Biogas and electricity are the most commonly generated forms of energy by any of the waste-to-energy processes.

At this stage, anaerobic digestion is the technology of choice for the conversion of poultry litter and other wastes into bioenergy and other bioproducts such as biofertiliser.

A challenge for the digestion of poultry litter, however, has been the need to add a large volume of water to digesters to adjust the litter to a total solids (TS) concentration of under 15% (as the TS concentration of poultry litter is about 70%).

The transition from dumping of poultry litter to repurposing of this waste is unescapable, but will require careful selection of technology. All of the existing technologies require a certain amount of investment; should a farmer choose an anaerobic digestion or thermochemical conversion option, the feasibility and justification would therefore need to be very strong.

Figure 1 shows the pathways a poultry farmer can choose to convert waste to energy and other value-added products.

An example of a chicken enterprise, showing energy, nutrient and revenue flows in the production system. The outputs are primarily biomethane, biofertilisers, bio-oil, syngas, and bio-char, which may be recycled or catalytically converted into further commodities such as hydrocarbons, chemicals, gasoline, diesel or BioCNG.

Composition of litter
Poultry litter comprises mainly bedding material, feathers, manure and wasted feed. The compositional characteristics of poultry litter are important to know, as they indicate several key physical properties such as moisture content, total (dry) solids, volatile solids, ash, and fixed carbon.

Analysis of litter content can also reveal other compounds, primarily protein, fat, non-structural sugars and fibres, whereas some poultry litter contains predominately cellulose, due to presence of the spent feed and bedding content.

This analysis indicates the potential of these building blocks in the waste materials. Where testing is not possible, data from pre-existing studies can be used instead.

  • Poultry litter with low TS waste
    The effects of using poultry litter as feedstock in anaerobic bioconversion in a conventional laboratory-scale or large-scale digester requires low-solids systems. These digesters allow for addition of significant volumes of water to bring the nominal organic loading rate to between 4% and 12% TS and a retention time of 30 to 40 days. Continuous stirred-tank reactors (CSTRs) are the most common digester configuration for this anaerobic treatment, and mixing the digested material throughout the process is crucial. Efficient mixing helps with mass transfer between the active microbial biomass and the litter.
  • Poultry litter with high TS waste
    High-solids anaerobic digestion offers many benefits, including smaller reactor sizing, lower heating, minimal mixing requirements, and decreased process water treatment. Plug-flow anaerobic reactors (PFRs) are characterised by minimal or no mixing and operating at high TS concentrations, which limits their applicability for several kinds of manure. PFR is known to offer flow through the reactor with no element of fluid overtaking or mixing with any other element ahead or behind. Moreover, PFR systems are usually simple, economical, and offer greater efficiency and overall bioconversion than CSTRs.

Heavy metal analysis

Evaluating the presence of heavy metals in poultry litter using an approved toxicity characteristic leaching procedure is highly recommended. Metal concentrations, for example, are normally determined by atomic adsorption spectrophotometry.

Although heavy metals are naturally present in soil, geologic and anthropogenic activities increase the concentration of these elements to levels harmful to plants and animals.

Due to the addition of various metals to poultry diets to facilitate weight increase and disease prevention, poultry litter generally has intensive quantities of heavy metals.

Hence poultry litter applied to large areas of land would result in heavy metal accumulation, posing potential environmental risks to the soil, surface and groundwater.

Biological methane potential analysis
Initial anaerobic digestibility assessments of the litter should be conducted employing the traditional biological methane potential (BMP) protocol, which uses a low-solids digester adapted to an inoculum from an existing anaerobic digester.

BMP tests have been used to characterise a wide variety of feedstocks for biogas production and are a key tool for investigating possible pre- and post-anaerobic digestion options. They use computer models and formulae that describe the anaerobic digestion process, hence the data produces reasonable predictions of full-scale anaerobic digester behaviour.

It is very important, therefore, to establish the methane potential for a given solid feedstock such as poultry litter and other vital parameters such as design and economic details of a biogas plant.

Ultimate analysis
Ultimate analysis of poultry litter provides the elemental composition of the major elements such as carbon, oxygen, hydrogen, nitrogen, phosphorus and sulphur in the sample. Research has shown the feasibility of using carbon and hydrogen contents of waste to estimate the heating value (HV).

Heating (calorific) value is a measure of the energy content in the waste. Two heating value types, namely higher heating value (HHV) and lower heating value (LHV), are found
in a waste sample. The HHV is the total amount of heat available in waste and includes the latent heat of vaporisation of water in the sample, while the LHV does not include this latent heat.

If a farmer plans to burn litter for energy in the presence of oxygen inside a sealed container, the heat released can be predicted by the heating value.

Hydrothermal liquefaction
Hydrothermal liquefaction (HTL) (hydro-pyrolysis) is a thermochemical conversion process in which high temperatures and pressures decompose organic material such as livestock waste.

It is a thermochemical conversion process, similar to pyrolysis, gasification or combustion, as all these use heat to decompose organic material into various products.

Studies of HTL performances suggest this could be a viable alternative to manage larger volumes of poultry litter effectively.

Laboratory-scale, pilot plant and commercial operations have produced a number of interesting products, including syngas (a mixture of water, hydrogen, carbon monoxide, carbon dioxide, nitrogen, and hydrocarbon gases), bio-oil (the highly oxygenated condensation product of synthesis gas), bio-char (a solid residual of thermochemical conversion), and water effluent (a nutrient-dense material with NPK showing potential as a biofertiliser).

Email Dr Idan Chiyanzu at [email protected], or Primrose Magama at [email protected].

- Advertisement -