Damascene conversion? Protecting our environmental heritage
A recent UN report, produced by its Environment Programme, confirmed that in excess of 1,000,000 tonnes of food waste is generated globally per annum and causes between 8 & 10% of all greenhouse gas emissions. First published in Energy Manager Magazine, Chartered Engineer Professor Robert Jackson and Partner Peter McHugh examine the changes required in attitudes, expectations and regulation to permit a sustainable balance between environmental burden and environmental capacity.
Although he was not one of the Twelve Apostles, the Christian apostle Saint Paul spread the teachings of Jesus in the first-century Anno Domini (AD). Born in Syria, around the same time as Jesus, he was a Greek speaking Jew who was converted to the Christian faith in or about 33AD and died in Rome circa 63AD. As one of the leaders of the first generation of Christians, he is often considered to be the most important person after Jesus in the history of Christianity.
Whilst travelling to Damascus he had a vision in the form of a blinding bright light where God revealed his Son to him. This revelation convinced Paul that God had indeed chosen Jesus to be the promised Messiah and it changed his life forever. Hence, on the road to Damascus Paul witnessed a seminal moment that led to a sudden and dramatic transformation in his attitude and beliefs.
Such a dramatic transformation and Damascene conversion is required in people’s attitudes and beliefs toward food wastage regarding its direct and profound implications to environmental pollution and human health. Interestingly, whilst AD has thus far denoted Anno Domini, in the context of environmental pollution AD denotes a most promising technology for food waste management in the form of anaerobic digestion.
Globally, the traditional disposal methods of large amounts of food waste have, to date, principally comprised landfill, incineration, and composting. However, these chosen methods have resulted in significant environmental pollution and increased financial risks. Annual world-wide food production currently stands at 2.7 billion tonnes with one third lost or wasted throughout the food supply chain.
By way of example, since 1974 the amount of food waste created in the United States alone has increased by 50% with 36 million tonnes of food waste now being discarded every year, whilst nearly 10% of its total population suffers from food insecurity. Currently, food waste equates to approximately 100kg per person and comprises the single largest component (22%) of municipal solid waste disposed to landfill. As a consequence, the food industry creates damaging social and economic side effects whilst simultaneously depleting the environment of limited natural resources; the problem is further compounded by the fact that in the US less than 2% of food waste is subject to AD.
Likewise, within the European Union an estimated 20% of the total food produced is lost or wasted, equating to approximately 88 million tonnes, whilst 33 million people cannot afford a quality meal every second day. 70% of EU food waste arises from households, food services and retail outlets, with households alone generating 47 million tonnes forming more than 50% of the total.
To assist in resolving this ongoing dilemma, anaerobic bacteria are able, by employing AD, to convert organic waste and biomass into biogas whilst leaving a nutrient-rich residue suitable for agricultural land use. AD is a process through which bacteria break down organic matter such as animal manure, wastewater bio-solids, and food waste, in the absence of oxygen. Biogas constituents typically comprise 60%-70% methane, 30%-40% carbon dioxide, together with traces of hydrogen and hydrogen sulphide. Hence food waste, being a high-moisture, energy-rich substance, may provide substantial economic benefits from AD by way of renewable energy production.
One of the principal advantages of AD over other bio-energy technologies is its ability to operate using a wide range of substrates i.e. those materials from which bacteria can obtain food and nourishment and on which they are able to grow and thrive. Food waste therefore comprises an excellent substrate for AD, due to its availability, quantity and high energy content.
During the anaerobic digestion of food waste, a mixture of gaseous compounds (biogas) is released that commonly includes odourless methane and carbon dioxide, together with ammonia and highly odorous volatile sulphur compounds which include hydrogen sulphide (H2S – odour of rotten eggs). Other odorous compounds emitted include ethyl mercaptan (C2H6S – garlic, onions, and cabbage); methyl mercaptan (CH4S – cheese); carbon disulphide (CS2 – rotten vegetables); and dimethyl sulphide ((CH3)2S – rotten vegetables and cabbage). All of these odorous gases are prejudicial to human health.
The benefits accrued from biogas generation using AD can often be further enhanced through chemical dosing to increase the biogas volumes yielded. This can be achieved by increasing the pH of the food waste. pH is a measure of the concentration of hydrogen ions in solution and is on a logarithmic scale from 0 (acid) to 14 (alkali). It is used to specify the acidity or alkalinity of an aqueous solution and a pH value of 7.0 is neutral, so a pH of 5.0 is ten times more acidic than a pH of 6.0 and one hundred times more acidic than a pH of 7.0.
Slow decomposition during prolonged low-pH conditions is a frequent process problem in food waste composting. However, artificially increasing the pH of food waste from 6.5 to 8.0 will increase biogas production by 10%, increase methane gas production by 65% and decrease hydrogen sulphide production by 45%. Hence, increased alkalinity increases methane and decreases hydrogen sulphide emissions but in so doing creates a potential source of toxic/explosive gas emissions. To illustrate this point, the microbial decomposition of organic matter in food waste can be demonstrated by the equation representing the decomposition of glucose: (glucose) C6H12O6 → (carbon dioxide) 3CO2 + (methane) 3CH4.
Constructing anything that is relatively innovative, and complex is full of risk. Even more so when constructing AD’s where chemical processes are at play. The level of risk in relation to possible death, personal injury or financial cost goes to a different level, indeed, references to the construction of AD systems often include alarming risks associated with: ‘asphyxiation’; ‘chemical hazards’; ‘gas or liquid leaks’; and ‘fire and explosion’.
AD for biogas production normally takes place in a sealed vessel comprising a reactor, which is designed and constructed for the specific site where it is to be located. Such reactors contain complex microbial constituents that break down (or digest) the waste and produce resultant biogas and dig-estate which is discharged from the digester. Consequently, the procurement of such complex plants needs to be carefully considered by the professional team engaged in their design and construction. A recent case: DBE Energy Limited -v- Biogas Products Limited, (DBE -v- BPL), highlighted the risks of failing to correctly design the equipment supplied to one such facility. This case comprised a claim brought by DBE for remedial costs and loss of revenue as a result of the catastrophic failure of digester tank heaters and pasteuriser tanks supplied by the Defendant, BPL, for inclusion at DBE’s anaerobic digestion facility in Dunsfold Park.
It was DBE’s case that failure of the equipment was caused by BPL’s negligence and/or breach of contract in designing the equipment. BPL was obliged to ensure that its equipment design could be safely integrated into, and would be compatible with, the overall design of a hot water system. However, BPL failed to carry out adequate structural design checks and testing to take account of the total maximum operating pressures. The equipment supplied by BPL was therefore unfit for purpose and BPL was negligent and in breach of contract.
It is imperative therefore to discuss legal and contractual arrangements with legal advisers and expert consultants at the project concept stage. Investing in appropriate contractual and technical advice at an early stage can greatly assist in project procurement and performance particularly in matters relating to plant ownership, funding, design, construction, delays, operation and insurance. Similarly, during the construction phase compliance with relevant legislation is imperative in terms of health and safety management, with primary legislation relating to health and safety risks during construction comprising the Construction (Design and Management) Regulations (CDM Regulations). Due to the nature and complexity of AD plants, all such projects will inevitably come under the auspices of these regulations and all parties are required to comply with their duties under the CDM Regulations; failure to do so risks legal action.
It is, therefore, prudent to reflect upon and learn from past events. On 23rd May 1984 a group of 44 visitors were at the opening of a new water supply project in Abbeystead, Lancashire commissioned by the then North West Water Authority. During a public presentation pumped water travelled up the tunnel/pipeline and displaced flammable gas that had accumulated in the tunnel from coal seams in the ground. Methane gas was pushed into the valve house where it ignited causing an explosion that completely destroyed the valve house causing 16 deaths.
The risks from methane were also recently illustrated by the explosion on 3rd December 2020 when a silo containing sewage sludge bio-solids exploded at a Wessex Water wastewater treatment works in Avonmouth, Bristol killing three men and a 16-year old apprentice. At the time, stored sewage sludge was being mixed with lime within oxygen-free tanks to produce agricultural fertiliser. Investigations by the Health & Safety Executive are ongoing but current thinking is that the explosion resulted from the anaerobic digestion of organic waste coupled with alkaline pre-treatment which increased methane production.
In conclusion, it is evident that the global drive toward sustainability has inherent risks and whilst regulation and legislation will undoubtedly have a profound effect upon attitudes toward consumption, resource depletion and environmental use, the key to success lies in the application of science.
