Anaerobic Digestion

Dairy production and renewable energy

Carbon Reduction At BV Dairy

Milk from the Blackmore Vale is processed by BV Dairy into fresh and cultured creams, soft cheese and yogurt, for sale to food manufacturers and food service customers.

BV Dairy is decreasing its carbon footprint (volume of carbon dioxide and other greenhouse gases emitted in manufacturing and distribution operations).  Emis1sions can be reduced by cutting fossil fuel consumption or generating renewable energy.  We are doing both – generating heat and electricity.

Clearfleau, our project partner, is a British company that designed the system for extracting energy from liquid food processing residues.  This plant was built with finance from the Environmental Transformation Fund (ETF), administered by WRAP (Waste & Resources Action Programme), as part of a Government initiative to stimulate innovative Anaerobic Digestion (AD).

Biogas From Processing Waste

Manufacturing products like soft cheese and yoghurt produces some waste materials, plus wash-waters from cleaning equipment.  Previously this was treated at the local sewage works and each week an additional 11 trucks of material were supplied to local pig farmers as animal feed (a declining market).

Bio-d2egradable materials, containing organic matter, sugars, fats and proteins, can be treated using AD.  In this natural degradation process, bacteria break down bio-degradable compounds in the absence of oxygen to produce biogas (mainly methane and carbon dioxide).

Digestion of this processing waste has required innovative process design.  Clearfleau’s system converts previously discarded materials into energy on a restricted footprint.  The process was successfully trialled here in 2009 – with a small scale plant, before building the main plant.

Product Objectives

This plant will treat the waste materials on site – thus reducing sewer disposal costs, while generating renewable energy for use in the dairy. Other key objectives for the project were to:

  • Minimise residual waste sent for treatment elsewhere
  • Produce biogas without adding other waste materials
  • Generate energy – both power and heat for use on site
  • Install it on a restricted footprint to limit local impact

WRAP’s aims in supporting the project are:

  • Cost effective production of biogas on industrial sites
  • Wider environmental benefits of anaerobic digestion
  • Reduction in the carbon footprint of food processing
  • More efficient treatment of industrial bio-effluents.

 

THE AD PROCESS – BIOMASS AND BACTERIA 

Anaerobic Digestion Feedstock

4Anaerobic Digestion (AD) is an established treatment process.  The bacteria break down organic material in the absence of oxygen.  Methane, carbon dioxide, water and a small amount of residual biomass are the by-products of this process.

Feedstock digestibility is measured using Chemical Oxygen Demand – expressed in milligrams per litre (mg/L).  This indicates the concentration of chemically oxidisable compounds – based on the mass of oxygen consumed, per litre of material during oxidation.

Clearfleau’s process creates conditions for bacteria to thrive.  Digestion is most efficient at a temperature of about 36°C.  The anaerobic reactor is heated using waste heat from an engine generator with a boiler as a back-up.  To start the digestion process the reactor tank is fed with bacteria.

Feedstock Supply

Plant washings and process by-products are pumped to the digestion plant. This stream contains milk protein, fats and other bio-degradable material that can be consumed by the bacteria.

This system is different to other AD plants because the feedstock is a relatively dilute liquid and due to its high water content, treatment using conventional AD systems is less effective.  The high-rate system can also handle the fats associated with dairy and other food processing waste.

Clearfleau’s reactor, designed for up to 200m3 per day of feed- stock, requires good mixing and constant feed to provide the bacteria with a regular food supply.  The feedstocks are pumped to two well insulated balancing tanks. From these tanks, material is fed into the AD reactor.

The Science of Anaerobic Digestion5

Within the anaerobic reactor tank, a variety of bacteria convert organic compounds into biogas.  There are four key stages in the AD reaction process:

  1. Hydrolysis: the feedstock contains both suspended and dissolved solids. Here the suspended solids are converted into soluble products, such as sugars, amino acids and alcohols.
  2. Acidogenesis: soluble organic products are converted into volatile fatty acids, plus by-products like ammonia and carbon dioxide. This “Acidogenic” (fermentative) biological reaction is like the process of milk going sour.
  3. Acetogenesis: this is a biological process where volatile fatty acids are fermented by “acetogenic” bacteria to produce acetic acid, carbon dioxide and hydrogen.
  4. Methanogenesis: slow growing “methanogenic” bacteria convert acetate and hydrogen, into methane and carbon dioxide – the main components of biogas.6

 

THE AD PROCESS – BIOGAS OPTIMISATION

Mixing and Heating

The slow growing anaerobic bacteria require a continuous food supply and a long solids retention time to avoid bacterial wash-out.  The process mixes different strength streams, providing a relatively constant feed to the reactor.

To minimise reactor tank volume, the process reduces the period for which liquids are retained in the tank but keeps bacteria in the system for about 50 days.  To retain biomass in the reactor tank the content is continually thickened.  Solids from the liquid outflow pass through the low energy thickening system and are returned to the reactor.

The bacteria thrive at a temperature of about 36°C.  Because rapid changes will inhibit bacterial activity, the reactor is kept at a consistent temperature by heating a recirculating stream from the reactor.7

Optimising Digestion

Anaerobic bacteria are among the oldest forms of life on earth.  They evolved in the period before photosynthesis of green plants released quantities of oxygen to the atmosphere.  Bacterial conversion of organic material facilitates natural decomposition processes.

In addition to maintaining appropriate nutrition, temperature and pH, optimisation of the biochemical reactions requires effective mixing in the reactor to ensure that the feedstock is accessible to the bacteria.  Effective reactor management will maximise biogas production and its methane content.

To enhance bio-solids breakdown and gas output, micro and macronutrients are fed into the reactor.  This sustains performance and prevents the content from becoming too acidic (i.e. avoiding the equivalent of indigestion!).  The control system allows continuous monitoring and effective management of the process and outputs.

8Process Outputs

Biogas is stored in the “bio-dome” (gas cover) above the reactor.  Some of the electricity generated from the gas, in a combined heat and power (CHP) generator, is used in the dairy, saving energy costs and reducing carbon emissions. The CHP also produces heat that is used in the AD process and the dairy.

Residual bio-solids (digestate) are dewatered and taken offsite for composting. Compared to aerobic treatment processes used for effluent treatment, AD minimises solids output as a result of much lower bacterial yield (biomass production).

The liquid portion is discharged to the sewer – equivalent in volume to the daily incoming flow but with a much reduced biodegradable load (COD).  The output COD is reduced by over 95%, compared to the incoming feedstock.

 

RENEWABLE ENERGY PRODUCTION9

Biogas Utilisation

Biogas produced in the AD plant consists of methane (60%-70%), plus carbon dioxide and trace levels of other gases (hydrogen, nitrogen and oxygen).  Prior to energy generation, the biogas is treated to reduce its moisture content and remove impurities.

Each kilogram of Chemical Oxygen Demand (COD) removed from the feedstock yields approximately 0.35 m3 of methane.  This plant can remove 3,000 kilos of COD per day.

The biogas is fed to the CHP engine, which generates renewable electricity for use in the dairy and supply to the national grid. Out-put is about 40% electricity, 40% recovered heat and 20% heat losses. Some heat is used to maintain the digester temperature.  The balance is transferred to the dairy for use in our production processes.10

Energy Efficiency

The CHP system (supplied by Ener-G) generates 2,150 MW of electricity and 1,685 MW of heat per annum.  This dramatically reduces reliance on fossil fuels for our energy needs.  As dairy output increases a larger CHP unit will be installed. Maximum anticipated output is 2,600 MW of electricity.

Surplus heat is being used in the dairy production process.  It is supplied through a heat exchanger and has replaced heat that was previously generated by oil burning boilers.

Production of renewable energy is supported by the Government through the Feed in Tariff system (FIT). FIT’s provide a cash subsidy for each kW of low carbon electricity.  The on-site energy production process used here can be replicated elsewhere in the food industry with a range of feedstocks.11

Carbon Reduction

There is concern about the “carbon footprint” of food production.  The carbon footprint of a business is the total amount of “greenhouse gases” produced, directly and indirectly, during production and waste treatment activities, usually expressed in tons of carbon dioxide (CO2) per year.

BV Dairy will cut its carbon footprint by about 60% – reducing its output of carbon by about 1200 tonnes per year.  This equates to the environmental impact of planting 120,000 trees.

As a pioneer of liquid digestion, using Clearfleau’s technology, BV Dairy will reduce its overall impact on the environment, 12whilst cutting production costs, as expected of food processors by Government, consumers and major retailers.  Companies that buy dairy ingredients from BV Dairy will also cut their carbon footprint.

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