The energy taken to treat the wastewater produced by one person each year will be anywhere from 18 to 100 kilowatt-hours. This is a huge range and impacts significantly upon operating costs. Where a site employs anaerobic digestion, the conversion of organic material into biogas in the absence of oxygen, much, but not all this energy requirement can be offset. Infrastructure and the type of assets in place influences the energy balance, but so too does the site’s operation and this is where a highly skilled workforce can have a big impact.

The scale of the energy opportunity

With ongoing infrastructure development and upgrading, the quantity of wastewater sludge produced in Ireland is projected to increase by 80 per cent by 2040 to 96,000 tonnes of dry solids. In addition to this, one million wet tonnes – equivalent to approximately 25,000 dry tonnes – of food waste are thrown away each year, enough to fill Croke Park more than twice over. Both wastewater sludges and food wastes are valuable sources of energy that can be unlocked by anaerobic digestion. Every tonne of dry matter digested contains between three and seven megawatt hours. Realising half of this by digestion would yield more than 230 gigawatt hours of renewable energy, as biogas. This equates to the energy consumed by 40,000 people each year.

The medium to long-term plan

Maximising energy recovery from sludges through anaerobic digestion is at the heart of Irish Water’s National Wastewater Sludge Management Plan: "There are 14 wastewater treatment plants in Ireland with anaerobic digestion currently in operation. More than 50 per cent of all wastewater sludge was anaerobically digested in 2014. This is expected to increase to approximately 65 per cent when WWTP upgrades, currently under way, are completed. It is proposed that the number of sites with anaerobic digestion is increased to 19, as the optimum strategy for treatment, with the biogas produced used for energy recovery in all cases. Advanced anaerobic digestion followed by reuse of the residual biosolids on land has been evaluated to be the most sustainable solution for wastewater sludge treatment and disposal."

Technology options are numerous

In addition to maximising returns from current assets, increasing population size and upgrading wastewater treatment plants, Ireland will be served by five new, additional digestion sites. These may incorporate so-called advanced digestion technologies which seek to increase the conversion of organics to biogas. Ireland is well placed to select from the best performing advanced digestion technologies, which have been robustly tested in the UK and elsewhere, and seen performances of >3MWh (in the form of biogas) for each tonne of dry solids processed on wastewater treatment sites. Digestion and advanced digestion are not, however, without their challenges, and it is here that experience is required to optimise and troubleshoot to achieve the goals of maximising energy production while achieving compliance at least cost. An experienced operator will spot the signs of a foaming digester before it foams, understand how and why to alter feed rates, be able to interpret biogas yield data, volatile fatty acid and alkalinity data and act upon these results.

Getting technical

One area that has generated much discussion over the past five years is ammonia inhibition in digestion and how this affects digester stability, revenue and throughput. Numerous water companies have in place a limit of 3,000 mg/l ammonium on advanced digestion facilities, where a site exceeds this the feed dry solids to the digester may be diluted. However, on food waste digestion sites it is not uncommon to see levels at more than 6000 mg/l, so why the concern and why the difference? Reduced nitrogen is present in two forms, the ammonium ion and unionised (also termed ‘free’) ammonia. Free ammonia is toxic to methanogens and the higher the pH the greater the proportion present, the two forms being in equilibrium at ~pH 9.3. Research and operational data show that the methane forming organisms can acclimatise, but equally the route to methane from intermediate compounds can be optimised. In a ‘normal’ digester the majority of biogas production comes from the conversion of acetic acid into methane and carbon dioxide, undertaken by so called acetoclastic methanogens. As free ammonia and pH increase the route to acetic acid and then to methane is compromised, where this happens the pathway to methane via hydrogen and carbon dioxide needs optimising. So-called hydrogenotrophic methanogens facilitate this reaction and are more tolerant to free ammonia than their acetotrophic cousins. The presence or absence of trace elements has been shown to help or hinder this route, thus many food waste sites now add trace elements to enable high ammonia operation, while some water companies are considering the cost-benefit. There is a choice for the operational team, reduce the nitrogen load to the digesters (and effectively limit throughout) or explore high ammonia/ammonium operation. The concentration in the digester should not however come as a surprise, it is a function of the digester performance (volatile solids destruction) and the total nitrogen content of the input material, both of which can be measured and used to populate predictive models.

Offsetting the costs of wastewater treatment

Where digestion is located at a wastewater treatment works, the energy required to drive the plant and meet consent levels is dominated by the aeration process, which will often be an activated sludge process. There are many different types of activated sludge process: oxidation ditches, lanes, sequencing batch reactors, membrane batch reactors, as well as the ‘new to the market’ granular sludge process, currently being installed at Dublin Ringsend. The energy required for aeration will account for 50-70 per cent of the total consumption and where digestion is present can be offset, providing an opportunity to move towards energy neutral treatment. To meet an effluent discharge consent for nitrogen, suspended solids or BOD (biochemical oxygen demand) at least cost, an experienced operator will understand how the age of the sludge affects effluent quality, that seasonal changes in the weather or population must be catered for and above all that decisions are made based upon accurately collected and measured data. What are the impacts of traders in the catchment? Is the recycling/disposal route for the digestate secure? Why has the sludge become odorous? When should I dose chemicals to treat the problem? What is the root cause? The list of questions an experienced operator asks is long and a measured approach to understanding the best course of action to take is perhaps the most valued trait.

Sludge age

Sludge age (also termed the Mean Cell Residence Time) and the F:M (Food to Mass or Micro-organisms ratio) arguably are the two most important design and operating parameters for activated sludge plants. They quantify the average age of the micro-organisms in the process and the amount of food that needs to be converted in to new biomass, on a daily basis. Longer sludge ages are required to remove ammonia from wastewaters, but the costs associated with increased aeration also rise. The sludge age will also affect the microbiology, which in turn affects the rate of settlement in the final sedimentation tanks. A site should therefore understand the optimum between sludge age, cost, effluent quality and settlement. There is no standard relationship for plants, low or high sludge ages can produce a well or poor settling sludge, depending upon the nature of the wastewater and plant configuration (plug flow or completely mixed). This difference or variability is greatest for industrial wastewater treatment plants (or those domestic sites with a high proportion of trade effluent). Having understood the optimum for any given site, the F:M can be manipulated to enhance settlement, by compartmentalising the process to select well settling micro-organisms (floc formers) over filamentous organisms. Creating a kinetic selector can be a cost-effective solution to settlement problems and where successful have replaced more expensive remedial options, typically construction of additional final tanks. A selector is not a guarantee and the mode of operation (aerobic, anoxic, anaerobic) and the floc loading required need careful consideration, as well as the ability to control the redox potential and return activated sludge load.

Experience and training count

Experienced, trained and well-supported operational teams are capable of investigation, acquiring new knowledge, learning from mistakes and introducing change in a measured, reasoned manner. Some knowledge of biology, chemistry, engineering, maths and microbiology is needed – and training can be provided in these aspects to provide a foundation. Good training aligns theory and practice, the result being process operators and scientists capable of optimising and troubleshooting.

Demand for science and engineering staff outstrips supply

Competition for experienced process operators, process engineers and process scientists is ferocious in Ireland. Ireland’s talent mismatch has been rated the highest around the world, and skills shortages exist in science and engineering. To deliver upon the country’s National Skills Strategy "talent cannot be left untapped... migrants will be encouraged to return to Ireland... and there will be increased mobility of early stage researchers. As we approach full employment, making sure Irish workers have the skills that enterprise needs matters more than ever." Government policy including Enterprise 2025, Pathways to work 2016–2020 and the Action Plan for Jobs focus on the importance of the Skills Agenda and are part of the Ireland’s strategy to deliver long-term sustainable growth. In the short term, knowledge gaps can be plugged by training and investing in existing staff, which has been shown to improve staff morale, loyalty and retention, but to be truly successful, training and development requires a long-term, regular commitment from employer and employee. Author: Matt Smyth is technical director at Aqua Enviro, part of Suez Water Technologies & Solutions. He has 20 years’ experience in the wastewater and anaerobic digestion sectors and has trained more than 1,000 people in these areas. He will be delivering a series of courses on behalf of Engineers Ireland in 2018, click here to find out more and book your place.