Visit to a waste treatment plant
Biofuels and electricity from waste
In the UK we are fast running out of landfill sites to dispose of our waste also as a member of the EU we are obliged to reduce the amount of waste that can’t be recycled.
Recycling allows precious resources such as copper, aluminium and rare earth metals to be re-used but biological matter has just as an important use.
We are very quickly running out of fossil fuels and we need a replacement especially for oil and gas. Biological matter can be used to produce biofuels (biogas and biodiesel).
A biofuel is a fuel that contains energy from geologically recent carbon fixation. These fuels are produced from living organisms. Examples of this carbon fixation occur in plants and microalgae. These fuels are made by a biomass conversion (biomass refers to recently living organisms, most often referring to plants or plant-derived materials). This biomass can be converted to convenient energy containing substances in three different ways: thermal conversion, chemical conversion, and biochemical conversion. This biomass conversion can result in fuel in solid, liquid, or gas form. This new biomass can be used for biofuels. Biofuels have increased in popularity because of rising oil prices and the need for energy security.
Bioliquids are liquid fuels made from biomass for energy purposes other than transport (i.e. heating and electricity).
Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl (methyl, ethyl, or propyl) esters. Biodiesel is typically made by chemically reacting lipids (e.g., vegetable oil, animal fat) with an alcohol producing fatty acid esters.
Bus run by biodiesel
Biogas typically refers to a gas produced by the breakdown of organic matter in the absence of oxygen. It is a renewable energy source, like solar and wind energy. Furthermore, biogas can be produced from regionally available raw materials and recycled waste and is environmentally friendly.
A biogas bus in Linköping, Sweden
Another aim is to produce electricity from biofuels
One of the best places for all these things to happen is a waste treatment plant.
A wastewater treatment plant (WWTP) or wastewater treatment works is an industrial structure designed to remove biological or chemical waste products from water, thereby permitting the treated water to be used for other purposes.
Sewage treatment is the process of removing contaminants from wastewater and household sewage, both runoff (effluents), domestic, commercial and institutional. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce an environmentally safe fluid waste stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or reuse (usually as farm fertilizer). Using advanced technology it is now possible to re-use sewage effluent for drinking water.
Typically sludge digestion is required after the aeration where the sludge may be unstable and requires further treatment. The specific purpose of digestion include:-
Stabilisation of solids
Reduce pathogenic organisms
Reduce volume of sludge
Obtain useful by-products
Two main methods of sludge digestion include anaerobic and aerobic digestion.
Aerobic & anaerobic
Aerobic occurs in the presence of free oxygen.
Anaerobic occurs in the absence of O2 and oxidised forms such as NO3, SO4 etc.
Aerobic vs anaerobic in sewage treatment
In aerobic sewage treatment you need to keep the ratio of bacteria to oxygen constant. This often means removing excess bacteria.
Anaerobic isn’t as efficient as aerobic. Aerobic needs less energy to deal with the quantities of sewage.
Hydrolysis takes about 2 days and methanogenesis takes about 11 to 13 days.
Grease and fats are high energy substrates.
Sewage and food waste can’t be mixed and trace elements have to be brought in for the benefit of the microbes.
Anaerobic digestion –
“The degradation and stabilisation of organic materials brought about by the action of anaerobic bacteria with the production of biogas.”
Anaerobic Digestion is favoured by the UK government as a method of producing fuel from biological waste and is being adopted by councils and food manufacturers across the country.
Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste and/or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion. Silage is produced by anaerobic digestion.
The picture below shows an anaerobic digestion and regenerative thermal oxidiser component of Lübeck mechanical biological treatment plant in Germany, 2007
The digesters work at 35oC and can reduce the quantity of e coli and salmonella.
Below are the key process stages of anaerobic digestion
The image below left is of an anaerobic digester
If it is possible then the carbon dioxide is stripped off and recycled.
Hydrolysis usually means the breaking of chemical bonds by the addition of water.
Fermentation is a metabolic process converting sugar to acids, gases and/or alcohol using yeast or bacteria.
Acetogenesis is a process through which acetate is produced by anaerobic bacteria from a variety of energy (for example, hydrogen) and carbon (for example, carbon dioxide) sources. The different bacterial species that are capable of acetogenesis are collectively termed acetogens.
Methanogenesis or biomethanation is the formation of methane by microbes known as methanogens.
Anaerobic photosynthesis, also known as anoxygenic photosynthesis, is the process by which certain bacteria use light energy to create organic compounds but do not produce oxygen. Anaerobes are those bacteria that cannot use oxygen to generate energy.
Anaerobic digesters offer certain advantages over aerobic digestion systems. The anaerobic process requires no oxygen supply and while mixing is desirable, the intensity required is not particularly high. In addition, the process produces methane which can be used as a source of energy within the treatment plant. However, the disadvantage is that anaerobic digesters usually require heating since the optimum temperature for successful digestion is generally about 35ºC or higher. Furthermore, the liquid separated from the solids is somewhat lower in quality when compared to aerobic systems.
Methanosarcina strain CHTI 55 grown on acetate. J. Ferry. Methanogenesis
In aerobic digestion, sludge stabilisation is achieved when aerobic and facultative microorganisms convert biodegradable organic matter in an environment where aeration is provided. End products of the digestion are mainly carbon dioxide, water and non-biodegradable materials.
In an aerobic system, such as composting, the microorganisms access free, gaseous oxygen directly from the surrounding atmosphere. The end products of an aerobic process are primarily carbon dioxide and water which are the stable, oxidised forms of carbon and hydrogen. If the biodegradable starting material contains nitrogen, phosphorus and sulphur, then the end products may also include their oxidised forms- nitrate, phosphate and sulphate. In an aerobic system the majority of the energy in the starting material is released as heat by their oxidisation into carbon dioxide and water.
The picture below left shows an aerobic treatment facility. The aerobic treatment was invented in 1914 by Arden and Lockett and known as the batch activated sludge process. Activated sludge is a process for treating sewage and industrial wastewaters using air and a biological floc composed of bacteria and protozoa. Aerobic bacteria need to be farmed off.
Above right is a picture of an aerobic digester
The major disadvantages are (i) higher power cost, (ii) solids produced have poorer dewatering characteristics, (iii) aerobic digestion is more temperature dependent since the digesters are mainly open tank systems, and (iv) there is no useful by-product generated such as methane.
Activated sludge is a process for treating sewage and industrial wastewaters using air and a biological floc composed of bacteria and protozoa.
The picture above shows a generalized, schematic diagram of an activated sludge process.
M A D = Mesophilic anaerobic digester
Mesophilic digestion takes place optimally around 30 to 38 °C, or at ambient temperatures between 20 and 45 °C, where mesophiles are the primary microorganism present.
APD process (acid phase digestion)
Acid Phase Digestion (APD) is reported to increase volatile solids destruction and gas production from anaerobic digestion.
Newer systems have two stage digestion APD/MPD. The old primary MAD acid phase (EH) is installed upfront. Secondary digesters are converted to primary MAD (E H is enzymic hydrolysis)
Acid Phase Digestion –Advantages
You can treat more sludge through the digestion process. There is a better breakdown of SAS (biological sludge). Enhanced sludge treatment (pathogen kill) occurs. More biogas is produced and secondary digesters for primary duty MAD are released.
What is the difference between SBRs and conventional aeration plants
Sequencing batch reactors (SBR) or sequential batch reactors are industrial processing tanks for the treatment of wastewater. SBR reactors treat waste water such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the waste water to reduce biochemical oxygen demand (BOD) and chemical oxygen demand (COD) to make suitable for discharge into sewers or for use on land. Aeration and settlement takes place in the same unit.
Below is part of an aeration plant. Usually a continuous process where aeration and settlement takes place in different units.
A batch process digesters need heat, which is provided by the CHP (combined heat and power), which needs gas which is provided by the digester.
Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to simultaneously generate electricity and useful heat.
Remaining sludge is limed.
The metazoa & protozoa that do the work
Better sewage treatment is the latest thing in clean energy.
Food waste produces more energy than sewage and can be used to generate up to 10 GWH of electricity p.a.
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible. The word is coined from the Greek-derived elements pyro “fire” and lysis “separating”.
The process is used heavily in the chemical industry, for example, to convert biomass into syngas and biochar, to turn waste into safely disposable substances, and for transforming medium-weight hydrocarbons from oil into lighter ones like gasoline.
Microbial fuel cells can use wastewater as a substrate
A microbial fuel cell (MFC) or biological fuel cell is a bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature.
The Renewables Obligation (RO) is designed to encourage generation of electricity from eligible renewable sources in the United Kingdom. It was introduced in England and Wales and in a different form (the Renewables Obligation (Scotland)) in Scotland in April 2002 and in Northern Ireland in April 2005, replacing the Non-Fossil Fuel Obligation which operated from 1990.
The RO places an obligation on licensed electricity suppliers in the United Kingdom to source an increasing proportion of electricity from renewable sources and suppliers meet their obligations by presenting Renewables Obligation Certificates (ROCs) to Ofgem.
A ROC is the green certificate issued for eligible renewable electricity generated within the United Kingdom and supplied to customers in the United Kingdom by a licensed supplier. ROCs are issued by Ofgem to accredited renewable generators. GENco has been issued with a ROC.
The Renewable Heat Incentive (RHI) is a new Government environmental programme that provides financial incentives to increase the uptake of renewable heat. For the non-domestic sector, broadly speaking it provides a subsidy, payable for 20 years, to eligible, renewable heat generators and producers of biomethane based in Great Britain. Ofgem is responsible for implementing and administering the scheme on behalf of the Department of Energy and Climate Change.
A feed-in tariff (FIT, standard offer contract advanced renewable tariff or renewable energy payments) is a policy mechanism designed to accelerate investment in renewable energy technologies. It achieves this by offering long-term contracts to renewable energy producers, typically based on the cost of generation of each technology.
Some bio-generators can earn a lot of money for generating electricity.
Photosynthesis and anaerobic processes in combination are used to turn a plant like grass into biogas.
Biochemical oxygen demand or B.O.D is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.
In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water.
A sewage plant’s major Components’ considerations
Total BOD, Total Suspended Solids, Chemical Oxygen Demand, nitrogen , Total Nitrogen, Total Kjhedal Nitrogen, Ammonia, Total Phosphorous, Temperature, pH
The Kjeldahl method or Kjeldahl digestion in analytical chemistry is a method for the quantitative determination of nitrogen in chemical substances developed by Johan Kjeldahl in 1883.
Total Kjeldahl nitrogen or TKN is the sum of organic nitrogen, ammonia (NH3), and ammonium (NH4+) in the chemical analysis of soil, water, or wastewater (e.g. sewage treatment plant effluent).
Today, TKN is a required parameter for regulatory reporting at many treatment plants, and as a means of monitoring plant operations.
BOD5, Organics, COD
Biochemical Oxygen Demand BOD: The amount of oxygen consumed during the biochemical reaction.
BOD5 is the measurement of the oxygen consumed during the biochemical oxidation of organic matter in 5 days at 20 oC – ATU (Allylthiourea) suppresses the ammonia Oxygen Demand
Organic Components – Typically Carbon, Hydrogen & Oxygen (& Nitrogen):
40 – 60 % Proteins, 25 – 50 % Carbohydrates, 8 – 12 % Oils & Fats, Traces of urea, Traces of synthetics organics
Chemical Oxygen Demand – All elements that can be oxidised chemically using dichromate in an acid solution with the exception of Ammonia
Quick Test – Becoming more widely used http://www.laboratoryequipment.com/product-releases/2007/11/rapid-cod-test-takes-two-hours
COD does not equal BOD ultimate because there are difficult Biodegradable Organic’s including lignin and inorganic materials interfere with the process.
COD – TBOD ultimate = Recalcient COD
The Ultimate BOD is the total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion.
Recalcient COD are materials that are difficult to break down, such as lignin.
The slowly biodegradable are those components which are high molecular size which will not diffuse directly through cell walls and therefore require hydrolysis to smaller molecules.
Nitrogen exists in the Water Cycle as: N bound into Organics; Ammonia/Ammonium (NH4+ NH3 + H+); Nitrates (NO3); Nitrites (NO2)
Classifications Normally Used: Total Kjhedal Nitrogen (TKN) includes Ammonia/Ammonium compounds and organically Bound Nitrogen; Total Nitrogen including TKN, nitrates and nitrites
Renewable energy generation
Each kg of organic solid contains roughly: 80% volatile or degradable matter; 20% inert matter.
During AD 50% of the volatile matter is destroyed (eaten by bacteria).
Every kg of volatile matter destroyed roughly results in 1 m3 of biogas.
Each m^3 of biogas contains approximately 60% CH4, 40% CO2 v/v.
In chemistry, the volume fraction is defined as the volume of a constituent vi divided by the volume of all constituents of the mixture v prior to mixing.
The gross calorific value (energy) of CH4 is 37 MJ/m3.
Therefore 1 kg of organic matter results in 3 MJ of energy.
Therefore 1 tonne of organic matter digested per day could power 600 light bulbs or 36 1 kW electric fires continuously or 80 households.
Gas clean up: Vehicle fuel or gas-to-grid? Assessing other uses of biogas. The aim for most companies is to become carbon neutral.
Rags can also be processed and 90% of the solid waste products can form fertilisers. In the past farmers had simply bought in chemical fertilisers. Now they can come to an arrangement with biowaste producers. This cuts down pollutants and the farmer’s expenditure.
Waste plastics can be also be used as an energy source as well as some of them being recycled to make new plastics.
The aim for the future is that everything can be recycled or re-used and the carbon footprint becomes zero.