Matt Smyth looks at the opportunity for advanced anaerobic digestion in Ireland, which, followed by reuse of the residual biosolids on land has been evaluated to be the most sustainable solution for wastewater sludge treatment and disposal.

Current and future picture for anaerobic digestion (AD) and advanced anaerobic digestion (AAD)

There are 14 wastewater treatment plants in Ireland with anaerobic digestion (AD) currently in operation. In 2014, these processed more than 50 per cent of all wastewater sludges, which will increase to 65 per cent once wastewater treatment plant upgrades are completed. Within Irish Water’s National Wastewater Sludge Management Plan, the number of AD sites will increase to 19, and by 2040 the total mass of wastewater sludge in Ireland is projected to increase by 80 per cent to 96,000 tonnes of dry solids. Advanced Anaerobic Digestion (AAD) is deemed the lowest carbon footprint and most economically feasible option for processing these sludges.

What does AD and AAD achieve and what is possible

Anaerobic digestion is the conversion of organics (also termed volatile solids) into biogas, which is largely composed of methane (about 60 per cent) and carbon dioxide (~40 per cent), plus digestate. The feed sludge organic fraction is approximately 75 to 80 per cent, the remainder being ash/inorganic solids, which are conserved in digestion. For sewage sludges, conventional mesophilic anaerobic digestion will destroy 30 to 40 per cent of the organics present, whereas advanced anaerobic digestion may achieve 60 per cent or more. Therefore, AAD will reduce the overall mass of solids more than conventional AD and produce a greater quantity of biogas. [caption id="attachment_42659" align="alignleft" width="300"] Figure 1 A basic mass balance illustrating the fate of one tonne of dry solids when processed by AD and AAD[/caption]  

Technology options

Broadly speaking, there are two options for AAD, both look to pre-treat the feed sludge; they are thermal hydrolysis and biological hydrolysis. In thermal hydrolysis, wastewater sludges are thickened to 16 to 18 per cent dry solids, then pressure cooked at temperatures of 160 to 170oC and 6-8 Bar pressure, before being rapidly depressurised. During this process, otherwise-difficult-to-break-down components of sewage sludge, especially SAS (Surplus Activated Sludge) are solubilised, by as much as 40 per cent. Injection of steam reduces the dry solids content to 14 per cent, which is then diluted further and cooled before feeding to the digesters, typically between nine and 11 per cent. Thermal hydrolysis is effective at killing pathogens and also greatly alters the sludge rheological properties; it is relatively easy to pump and mix, even at high dry solids concentrations. The sludge connoisseur will also notice a distinctively different odour associated with the sludge, almost hedonic with caramelised undertones, a hint of grandfather’s desk and barbecue on the beach; all of which is a product of the cooking process and the intermediate compounds formed. Tasting, however, is not advised, however tempted you may now be. Biological hydrolysis also looks to solubilise the feedstock and produce intermediate compounds, termed volatile fatty acids, achieving concentrations as high as 10,000 mg/l, more typically 4,000-6,000 mg/l. Without the high temperature cooking, though, the olfactometry sensors are less pleased, with a successfully biologically hydrolysed sludge being more akin to a mixture of pickling juice, sweaty clothes and vomit. While this may sound unpleasant, this is actually a good sign and knowing your nose is one of the most reliable methods for process scientists and operators to identify well or underperforming plants. Biological hydrolysis achieves the pathogen kill that thermal hydrolysis is so effective at by holding the sludge at 55-600C, typically for five hours, in batches. The plants are configured to give excellent plug flow characteristics, which minimises short circuiting, and allows optimisation of the initial phases of digestion, hydrolysis, acidogenesis and acetogenesis. The energy input for biological hydrolysis is less than for thermal hydrolysis and the energy output is usually similar on average, but it has historically been difficult to understand why one plant with seemingly the ‘same’ feedstock may perform better than another.

Energy content of sludge and energy availability

Usually the energy produced from digestion is expressed as kilowatthours (kWh) or megawatthours (MWh) per tonne of dry solids processed. The majority of sites in the UK and Ireland convert the biogas produced into electricity and heat through a combined heat and power engine and it is these values that are often reported to energy teams. However, in the drive to maximise energy from digestion it is increasingly necessary to understand energy in the gas phase, in the digestate, in the feed to the digester and the availability of that energy to the AD process. As an industry we have known for many years that primary sludges can often have both a higher energy content and higher energy availability than secondary activated sludges, and that advanced AD increases energy availability, but rarely is the difference quantified for different sites or sludges. If we are to move towards a future where energy is maximised, this is a step change that will need to be taken. To explore this further, a business could firstly measure the energy content in its different sludges, which may vary from as low as 4.5 MWh to as high as 7 MWh/TDS. For different feed sludges, with a measured (by way of example) energy content of 5.5 MWh/TDS (figure 2) and being subjected to the same AAD process, the difference in energy produced is down to the sludge characteristics and how available that energy is to the AAD process. [caption id="attachment_42660" align="alignright" width="300"] Figure 3 The energy generated from AAD is dependent upon the biodegradability of the feedstock, being much higher where biodegradability is similarly high[/caption] Current industry leading performance is 1.2 MWhe/TDS and many water companies would be ecstatic at this energy production as the ‘average’ site will more likely be in the range 0.8-1.0 MWhe/TDS, but more is possible, perhaps much more, 1.6-1.8 MWhe/TDS.

'How?'

Our understanding of 'how' is growing and this knowledge could begin to inform future investment decisions, favouring those that increase the total energy content and its availability to AD or AAD. Test methods have been developed that allow the user to fractionate the energy into four forms or more, which include: biodegradable particulate, biodegradable soluble, non-biodegradable particulate, and non-biodegradable soluble. This data, along with a site’s operational parameters, flows, tank sizes and configuration can then be fed into models, calibrated against a site’s current performance and then simulations developed with different feedstock characteristics that improve or reduce energy generation. [caption id="attachment_42667" align="alignright" width="300"] Figure 4: An example of model developed to simulate scenarios whereby energy generation can be increased[/caption] The simulations show the user what is possible, but also permit different feedstock inputs across the business to be ranked in terms of energy content and availability and ultimately revenue in euros/TDS. This information could in turn inform tankering operations, ensuring that the most valuable sludges are directed to the most energy-productive AD sites.

Innovation ready

The modelling also assists users in determining how efficient a given site is, which is apparent where the model’s energy output is far greater than actual performance. Any site with a shortfall is prime for an improvement project, whereas a site that is achieving the model output is maximised and potentially a suitable candidate for innovation projects. Trials can be taken on with the confidence that any gains are attributable to the innovation and not one of the many other factors affecting day-to-day operations.

Advanced AD for Ireland

AAD looks set for Ireland and, like the UK, looks to be the preferred technology choice for the current and next generation of engineers, process scientists, managers and operators. There are many plants in operation and dozens of case studies to learn from. Since the first AAD plant was installed 20 years ago, many of the operating experiences have been presented at Aqua Enviro’s European Biosolids & Organic Resources Conference which, in November, will run for the 22nd time in Leeds, to be held at the Royal Armouries, from November 13-14. Meanwhile, ‘Optimising Anaerobic Digestion Plants’, will be held at Engineers Ireland, 22 Clyde Road, on Wednesday, May 23.

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, including ‘Optimising Anaerobic Digestion Plants’, on Wednesday May 23, at Engineers Ireland, 22 Clyde Road.