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; this 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.
Notwithstanding these commendable aims, the renewable energy sector is littered with projects where negligence and/or breach of contract has arisen. This can result from either the designer or the contractor failing to honour the contractual terms and/or failing to act with reasonable skill and care. The construction of an EfW plant can be a huge undertaking requiring the right team of experts to plan and build. Design-and-Build contracts place a heavy burden on the Contractor who will become liable for matters contractual, technical and operational. EfW plants are not simple production facilities but have great complexity akin to that of ‘mini power stations’ requiring careful design and construction by specialist contractors.
Increasingly, with advances in technology and innovation, new plants can be more cost effective, with enhanced safety and improved environmental benefits. But, failure to design and construct properly can lead to a magnitude of problems for the contractor and developer, as well as greater risks to plant operating staff and the wider general public. The construction of these projects remains ‘complex’ and is often confronted with disruption and disputes. Extra design costs and time delays can have a profound impact on the critical path of build activities and budget planning. Multiple sub-contractors are often required due to the interdisciplinary nature of waste projects and EfW plants are challenging and high risk for contractors, engineers, architects and designers alike. A common form of contract in this sector is the ‘Engineering, Procurement & Construction’ (EPC) contract which permits the plant owner to manage risk more effectively. The structure of the EPC allows both project owner & contractors to mutually benefit from this type of contractual agreement by assigning risks to ensure the designed output is achieved whilst administering responsibilities and liability.
For the procurement of a UK-based waste plant comprising an anaerobic digester, a contractor appointed a consulting engineer to develop and complete the design of the process engineering elements of the plant. The consulting engineer completed a detailed design but chose to provide a design which went beyond what was required in the ‘Delivery Plan’ thus adding to the contractor’s cost. It was held by the court that the designer was liable for the contractor’s increased costs. The court declared that a contract appointing a design consultant to develop and complete a waste treatment process required the consultant to comply with specific design and delivery obligations insofar as they accorded with the overriding objective to take reasonable skill and care. Under the contract, the engineer was prima facie liable for the cost consequences of any modifications to the design which did not comply with the specification or ‘Delivery Plan’; (MW High Tech Projects v Haase Environmental Consulting  EWHC 152 (TCC)). Designers of EfW plants are therefore in the firing line when it comes to faults in the design, and consequential losses due to increased operating costs, lost income generation from energy sales, and negligence leading to death or injury. EfW plants often rely on the generation of methane as an energy source, but the chemical properties of this gas make its handling inherently troublesome. This difficulty is perhaps well illustrated by reference to a case in the early 80’s.
In 1984, in order to satisfy an increased demand for local drinking water supplies, a new water supply project in Lancashire was commissioned by the then North West Water Authority (NWWA). This required the daily extraction of fresh water from the River Lune and its transfer, by pumping, to the River Wyre. The Lune/Wyre Transfer Scheme comprised the construction of screen facilities at the water intake from the Lune, a pumping station, a transfer tunnel excavated through carboniferous limestone, a valve house in Abbeystead, and an outfall to the Wyre.
During the excavation of the tunnel, there had been a considerable in-flow of groundwater through the tunnel walls which continued to take place even after the rock tunnel had been lined with concrete. Also during excavation, traces of flammable natural gas were detected but these were deemed to be sufficiently low for the tunnel to be considered gas-free by normal tunnelling standards. Furthermore, the tunnel was designed to run full of water during pumping and to remain full with the pumps turned off. Consequently a feature of the design was that during standstill periods from pumping the tunnel would remain full of water to stop the ingress of groundwater and any gases emanating from the ground. The conditions of contract required the concentration of flammable contaminants to be kept below 10% of their lower explosive limit and there was no concern at the design stage regarding flammable gases being encountered during tunnelling.
On 23rd May a group of 44 visitors, comprising 36 residents of the local village of St. Michael’s-on-Wyre and 8 employees of NWWA, assembled inside the underground valve house set into the hillside at the outfall end of the scheme to attend a public presentation. This was to demonstrate the operation of the pumping station but when the pumps were started the pumped water travelled up the tunnel and in so doing displaced flammable gas that had accumulated in the tunnel from the ground. The gas was pushed into the valve house where it ignited causing an explosion that completely destroyed the valve house causing 16 deaths. No one escaped without injury, 8 were killed instantly by the explosion, with another 8 people, including an 11-year-old child and his mother, subsequently dying from their injuries in hospital. Every member of the visiting party was either killed or injured, with many suffering from severe burns.
The flammable gas was later identified as methane and its source, determined through geological and seismic surveys, was found to be coal seams located at a depth of 1.2km beneath the tunnel. The gas had collected over millions of years in a natural limestone reservoir from which it seeped towards the ground surface through a complex network of geological faults. The excavation of the tunnel had subsequently intersected these faults and allowed the gas to seep into the tunnel through its concrete lining. The cause of ignition has never been determined.
The Abbeystead disaster is an example of the responsibility on the design team to consider all possibilities including the presence of methane, and investigations later found that the possibility of a methane-rich environment had not been recognised. The fact that large quantities of methane might be dissolved in groundwater and subsequently released into the atmosphere within the tunnel was not considered by those involved with the design/operation of the system.
In 1987, at Lancaster High Court, the consulting engineers were found to be 55% liable in negligence for failing to exercise “reasonable care” in assessing the risk of methane. NWWA was found to be 30% to blame for failing to ensure the plant was safe for visitors and employees by testing for methane, and the contractor who constructed the works was found 15% liable for failing to carry out systematic tests for methane. All three parties appealed and a case was brought by the victims which went to the Court of Appeal. In the case (Eckersley and others v Binnie and others  18 CON LR 44, CA) negligence claims were brought by the survivors and relatives of the deceased against those responsible for the design, construction and operation of the works. Bingham L J stated when referring to the first defendants, the consulting engineers, that: “the original trial judge was entitled to find on the evidence that there was a risk of methane being present which should be taken into account in the design………..”.
The court of appeal held when considering the duties of care owed by each of the three defendants that in relation to the design stage, the consulting engineers who were the first defendant are expected to exercise the skill of a reasonable competent engineer specialising in the field of construction. The court held that in the circumstances, a reasonably competent engineer specialising in the design of water systems ought to have detected a risk of methane being present in the aqueduct. The explosion was therefore reasonably foreseeable.
The consulting engineers were held to be negligent in failing to consider the possibility that methane may be present when designing the aqueduct. The second defendants, the contractor, the court found was in breach of its contractual duty to test for methane, but this did not give rise to breach of any duty of care to the plaintiffs and the third defendants, the local authority, the court held that the evidence did not disclose any actionable negligence on its part. So, ultimately the consulting engineers were found solely liable. Leave to appeal to the House of Lords was refused and in February 1989 most of the injured survivors and relatives of those who died accepted out-of-court settlements from the consulting engineers.
In conclusion, sustainable development can be defined as ‘development that meets the needs of the present without compromising the ability of future generations to meet their own needs’. Whilst this definition is commendable, it is something of an oxymoron in that we have little or no insight into anything more than the very basic needs of future generations. Moreover, this article clearly highlights the imperative of understanding the environmental impacts of the past on present and future infrastructure development and if calamities are to be avoided then there is a need to recognise that ‘conjecture is not expert opinion’ and ‘hope is not a credible tactic’.
Professor Robert Jackson
Chartered Civil Engineer, Accredited Mediator for Civil & Commercial Disputes, Law Society Checked Expert, Forensic Engineering Expert in Water, Energy, Waste, Construction & the Environment. JACKSON Consulting
M: 07976 361716 E: email@example.com
Solicitor, Chartered Arbitrator & Accredited Mediator, and Specialist in Construction Dispute Resolution. Partner at Clarke Willmott Solicitors