The Chemistry of Conflict: Incendiary Disputes – Part 1

In the first half of this paper, Forensic Engineering Expert Professor Robert Jackson and Construction Lawyer Peter McHugh discuss the risks to sustainability borne from the explosive consequences of energy-from-waste (EfW) facilities. The paper will highlight contentious disputes relating to design, construction, operation, environmental impact and personal injury, together with strategies for their avoidance.

A timeline through the ages, via the Middle East, Italy, India and the UK, allows the scene to be set to discuss ongoing challenges to the creation of a sustainable future. In the 10th Century BC, bio-gas, a mixture of gases emitted from natural waste decomposition through the anaerobic breakdown of organic matter, was first used to heat bath water in the Middle East. In the 17th century Robert Boyle, one of the founders of modern chemistry and best known for Boyle’s law, recognised that disturbing settled sediments within natural waters released a flammable gas. In the late 18th Century the Italian physicist Alessandro Volta identified this gas to be methane and in the early 19th Century, Sir Humphry Davy, the inventor of the Davy lamp for use in flammable atmospheres, proved the presence of methane in gases emitted from cattle manure. However, it was not until 1859 that the first commercial anaerobic digester was constructed, albeit at a leper colony in Bombay. Since then much has changed and, in today’s global drive toward sustainability, UK waste streams are becoming an increasingly important source of energy. Modern society produces wastes in many forms but this paper will focus on two topical processes: the anaerobic treatment of wastewater and the incineration of solid wastes.

During the anaerobic digestion of wastewater, a mixture of gaseous compounds (biogas) is released into the atmosphere which commonly includes odourless methane and carbon dioxide, together with ammonia, and highly odorous volatile sulphur compounds contained within human sewage which include hydrogen sulphide. Hence the mixture is made from the individual elements of carbon, hydrogen, oxygen, sulphur and nitrogen. The resulting development of odour nuisance depends on individual odour characteristics, odour dispersion and dilution, together with peoples’ perception, and its environmental impact can be determined by assessing its frequency, intensity, duration, offensiveness and location. Emitted odorous compounds, other than hydrogen sulphide (H2S – rotten eggs), include ethyl mercaptan (C2H6S – garlic, onions, and cabbage) and methyl mercaptan (CH4S – faeces and cheese), all of which are prejudicial to the health of people living or working in the affected areas. Human excreta, comprising faeces and urine, also contain pathogens and viral and bacterial toxins.

pH is the scale used to specify the acidity or alkalinity of an aqueous solution and is on a logarithmic scale of 0.0-14.0, with zero the most acidic and 14 the most alkaline. The halfway point of 7.0 is neutral with acidity increasing as pH decreases from 7.0 to 0.0, and alkalinity increasing as pH increases from 7.0-14.0. As the scale is logarithmic, each pH value below 7 is ten times more acidic than the next level, so a pH of 5 is ten times more acidic than a pH of 6 and one hundred times more acidic than a pH of 7.

Chemical and biological reactions in sewage greatly depend on the pH with sewage treatment processes normally operating best within the neutral/alkaline pH range of between 7.0 and 8.0, whilst ammonia oxidising bacteria for nitrification prefer 7.2 to 8.2. Raising the pH is usually carried out by adding sodium hydroxide (caustic soda) or sodium carbonate to the incoming sewage flow. These compounds yield moderately alkaline solutions in water and are used as pH regulators to maintain stable alkaline conditions.

Domestic sewage often comprises food waste residues which may include by-products of glucose contained within, for example, cakes, pies, honey, bananas, ketchup and fruit juices. When sewage undergoes organic decomposition it produces a biogas that typically contains 50-75% methane; 25-45% carbon dioxide; 3% hydrogen sulphide; <2% nitrogen; and <1% hydrogen; plus trace amounts of other gases. An example of this chemical breakdown through anaerobic digestion can be illustrated by the equation representing the decomposition of glucose: (Glucose) C6H12O6 → (Carbon Dioxide) 3CO2 + (Methane) 3CH4.

Hydrogen sulphide is the most common form of volatile sulphur in faeces which have an average pH of 6.6 i.e. raw human sewage is slightly acidic. However, percentage gas emissions vary and high pH values inhibit the activity of sulphate-reducing bacteria. So, increasing the pH from 6.5 to 8.0 increases biogas production by 10%, increases methane production by 65%, but decreases hydrogen sulphide production by 45%. Therefore, increased alkalinity increases methane emissions and decreases hydrogen sulphide emissions.

Consequently, it is undeniable that wastewater treatment plants remain a potential source of toxic/explosive gas emissions from anaerobic digestion following the microbial decomposition of organic matter in sewage. Methane, together with carbon dioxide, ozone and nitrous oxide, is a major greenhouse gas responsible for global warming but is also the main constituent of natural gas. Hydrogen sulphide is a highly flammable irritant/asphyxiant that is heavier than air and so may travel along the ground, often collecting in low-lying, enclosed and poorly-ventilated areas including manholes and sewers. If mixed with air the gas may become explosive, and if it is able to travel to a source of ignition it burns to produce toxic vapours which may include sulphur dioxide. Furthermore, gases emitted from anaerobic digestion have a direct and undeniable impact on nuisance and human health; repeated exposure, even at low concentrations, often results in irritation to the eyes, nose, throat and respiratory system with prolonged exposure leading to headaches, conjunctivitis, insomnia, irritability, digestive problems, fatigue, central nervous system weakness, or weight loss.

Whilst methane lends itself to being a sustainable and renewable source of energy, its capture and storage prior to use is technically challenging for the designers, constructors and operators of energy-from-waste (EfW) facilities. This is shockingly demonstrated by the recent 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, and injuring another person. At the time, stored sludge was undergoing further treatment by mixing it with lime within oxygen-free tanks to produce agricultural fertiliser and renewable energy. Investigations by the Health & Safety Executive (HS&E) are ongoing but current thinking is that the explosions resulted from the anaerobic digestion of organic waste coupled with alkaline pre-treatment which increased methane production. The bio-methane gas produced by the works was supplied to local bus operators including Bristol Community Transport responsible for a Metro-Bus route.

Such an explosion will push air outwards at very high speeds creating a partial vacuum in its wake that will subsequently be filled with air from the surrounding atmosphere. This reaction will create an abrupt change in pressure which can give rise to a powerful shockwave and blast of air which, under certain circumstances, may have fatal consequences. This supports the need for wastewater treatment works and municipal waste incinerators to be constructed and operated at a safe distance from private housing, to eliminate potential risks to human health and safety emanating from anaerobic gas emissions.

The findings of a study recently published in the journal The Proceedings of the National Academies of Science show that whilst hydrogen sulphide gas is poisonous, corrosive, and smells of rotten eggs, it may help protect ageing brain cells against Alzheimer’s disease. The research showed that the human body naturally creates small amounts of hydrogen sulphide to help regulate functions across the body, from cell metabolism to dilating blood vessels. It is very important to remember, however, that the beneficial effects of hydrogen sulphide are most probably generated by exposure to that gas in pure form and on its own, without the other compounds present in emissions from wastewater treatment works.

In parallel with risks borne from an engineering and scientific perspective, there is an imperative to address the financial and legal implications of the development of EfW plants within the wider economy. A circular economy is an economic system aimed at the continual use of resources by employing systems of recycling, re-use, re-manufacturing and refurbishment to create a closed system that minimises resource input and the creation of waste, whether municipal, household, biodegradable or agricultural. However, part of minimising the creation of waste involves the transition to renewable energy from waste projects.

EfW is a sector that can be fraught with problems not dissimilar to those faced by the pioneers of the 19th Century Gold rush when failed schemes resulted in the loss of money, confidence, reputation, or injury. Whilst operating an EfW plant will yield great benefits for the environment and financial rewards for the pioneer owners, excessive build costs and associated risks are areas that need to be eradicated. In a modern society where climate change is of paramount importance, the role that EfW plants play cannot be underestimated with respect to reducing our carbon footprint and supporting the Covid-19 recovery.

The EfW sector is fast-growing and driven by changes in legalisation and regulation that have taken place over the last 20 years, to reduce landfill and pollution whilst increasing the generation of low carbon energy. In June 2019, the UK parliament passed legislation requiring the government to reduce the UK’s net emissions of greenhouse gases by 100% relative to 1990 levels by 2050. Doing so would make the UK a ‘Net Zero’ emitter by achieving a balance between the amount of greenhouse gas emissions produced and the amount removed from the atmosphere. There are two different routes to achieving ‘Net Zero’, which work in tandem: reducing existing emissions and actively removing greenhouse gases. Consequently, EfW will play an important future role in the future economy and will offer a potentially lucrative opportunity for UK-based contractors and professionals.

The Authors: