Sustainable developments in the use of anaerobic digestion to treat wastewater
09 January 2018
Growing concern over the years has demonstrated that the use of conventional, albeit well proven, aerobic biotechnology to treat wastewater is prone to production of non-viable quantities of excess sludges (more than 10 times the projected mass from best available technology), loss of nutrient from all phases, increase in carbon footprint relating to energy outlay and overdependence on chemistry versus biology.
Fundamental R&D and full applications of anaerobic digestion (AD) have been growing since the early 1800s and the challenging areas to be addressed when applying this expanse of knowledge are:
- Fossil-fuel reduction while maximising energy recovery;
- The correct selection of proven turn-key technology;
- Correct strategies in relation to feedlot mixtures;
- Proper conditioning and mixing of feedstock inputs;
- Reuses of products resulting from the AD – if necessary after further conditioning;
- Decisions on most appropriate use(s) of biogas;
- Nutrient (N) recovery for recycling with minimum pathogen carryover in digestate humus.
AD technology has become a norm across Europe and other areas of the world, but has yet to be properly developed in other regions – not least Ireland, even though some brilliant research with associated installations was carried out here as far back as the early 1980s while maximising energy recovery.
Proper characterisation of the feedstock(s) is essential to confirm the lead design parameters which dictate scale and efficiency of the reactor in terms of organics loading, detention, retention, reduction efficiency (COD v VS), and specific biogas production and biogas quality.
AD design standards have been modelled in the International Water Association publication AWM No1, utilising the cradle-to-cradle principle shown in Figure 1.
The saleable products are steam, other heat outlets, electricity, soil nutrients/conditioners, compost etc. Energy crops and liquids (milk, etc) can be used for AD in place of animal feed. In this case, the aim is to produce as much biogas as possible and a good quality soil nutrient and humus conditioner.
Commercial evaluation will define viability of this concept at masterplan stage. The most valuable application of AD combines both waste management and by-products use and, using the AD process, convert the waste(water) ‘liability’ to a series of ‘assets’ – such as biogas, nutrients, humus compost – which, when properly managed, will demonstrate the commercial benefits of this new strategy.
Four stages of the anaerobic process
The AD process can be further divided into four stages: pre-treatment, digestion, gas upgrading and treatment for re-use of digestates and side flows.
The level of pre-treatment depends on the type of feedstock. For example, liquid wastewaters, manures are sludges need to be mixed, whereas municipal solid wastes (MSW), grasses and botanic wastes are pre- sorted and shredded.
The digestion stage takes place in the AD reactor(s). There are many types of digesters with different temperature, mixing devices, etc. The digestion can be either dry or wet depending on the solid content. Thus, the feedstock can be mixed with waters and other appropriate liquid wastes such as sewage sludge liquid organic wastewaters or re-circulated liquid from the digester effluent.
‘Mainstream’ digestion can perform ideally as a wastewater pre-treatment operation, removing up to 90% of COD with less than 20% of the equivalent excess sludge production.
Biogas produced during the digestion stage requires removal of hydrogen sulphide and water vapour. These can damage boilers or engines and should be removed. Removal of carbon dioxide will be required if the gas is to be used as natural gas or vehicle fuel. Choice of system is project based but efficiency of energy conversion should not be forgotten e.g. steam (95%), CHP (55%), engine (40%), fuel cell (50%+). Flaring is a safety inclusion, but otherwise does not provide reuse.
The AD process is a naturally occurring adiabatic biochemical process of decomposition and decay, by which organic matter in any bio treatable condition is broken down to its end products under anaerobic conditions.
Anaerobic microorganisms digest the organic materials, in the absence of oxygen, to produce methane and carbon dioxide and other end-products under ideal conditions. The biogas produced in AD plant usually contains hydrogen sulphide (H2S), ammonia (NH3) and carbon dioxide (CO2), as well as trace amounts of other gases. Concentrations vary with the feedstock constituents.
The science underlying AD can be complex, and the process is best understood if split into the three main stages: hydrolysis, acidogenesis and methanogenesis. The AD cultures are well established from microbiology studies over the years but ongoing development of new cultures is necessary (see cited references).
During the hydrolysis, the fermentative bacteria convert the insoluble complex organic matter, such as cellulose, into soluble molecules such as fatty acids, amino acids and sugars.
In the second stage, acetogenic bacteria, also known as acid formers, convert the products from the first stage into simple organic acids, carbon dioxide and hydrogen. The principal acids produced are acetic acid, butyric acid, propionic acid and ethanol.
Finally, methane is produced during methanogenesis by bacteria called methane formers. The acetate reaction is the primary producer of methane because of the limited amount of hydrogen available. It is important to note that some organic materials, such as lignin, remain effectively undigested, as of course do non-organic inclusions within the waste. This can bias feedstock choices if considering high bio-energy production performance.
Conditions and variables influencing AD
Conditions and variables influencing AD must be considered in order to obtain a proper breakdown of the organic compounds. The operating parameters of the digester must be controlled so as to optimise the microbial activity and thus increase the AD efficiency.
Some of these parameters are discussed briefly below and others are noted for study in the main publication. The variables are:
- Total solids: these are three different reactor solids working ranges:
- Low solid (LS) AD systems contain less than 10% total solid (TS);
- Medium solid (MS) systems from 15-20%;
- High solid (HS) systems range from 22-40%.
- When increasing the total solid content, the volume of the digester decreases and mixing intensity increases, giving benefit in terms of energy conservation.
- Mixing: mixing within the digester improves the contact between the micro-organisms and substrate and improves the bacterial population’s ability to obtain nutrients. Mixing also prevents the formation of scum and the development of temperature gradients within the digester. However, excessive mixing can disrupt the microorganisms and therefore slow mixing is preferred. In case of co-digestion, the different feedstocks should be mixed before entering the digester to ensure a sufficient homogeneity. High-rate UASB/GSB designs maintain tank turnover using the particulate solids bacteria rise rate assisted by gas-lift to achieve mixing.
- Process design of reactors is normally based on the biological capacity of the reacting mass to convert COD or volatile solids (VS) to a reacting biomass, i.e. kgCOD (or VS) per m3 reactor as a nominal unit loading rate. When taken in combination with retention time and the biodegradability of the substrate (feedstock), the performance efficiency and allied biogas production may be predicted. Typical organic loading rate (OLR)/retention/efficiencies are:
|OLR||6.0-14.0 kgs COD/m3d||3.0-10 kgs VS/m3d|
|Hydraulic Detention||0.3-0.5 days||5.0-40 days|
|Conversion Efficiency||80-90% COD||50-65% VS|
Note: the above highly dependent on feedstocks, COD/VS ratios and biodegradability
Like any design combining operations of treatment, the conditioning stages concluded necessary determine the ultimate success of the development for example, removal of components of toxic or unacceptable constraints, physical and/or chemical conditioning, mixing and producing a controllable feed regime to ensure optimised reactor performance.
Pre- design characterisation determines the economics of the AD development. This area of system design and planning is even more important than basic reactor designs, which have been perfected by many specialists over the years.
Typical issues to be addressed at pre-conditioning stages are:
- Animal-based feedstocks where the EU regulations (Animal By-Products 2014) apply;
- Selection of single, or multi-stage and reactor type – which, again, depends on feedstock treatability, working temperature, the mixing concept and the prime performance objectives of the working process i.e. energy generation, sludge vs destruction, coliform limitations on the recycled product and mainstream biotreatment/bioenergy production;
- Agricultural co-product feedstock and mainstream wastewater feedstock giving bio-energy recovery at minimum sludge production;
- Digestion of MSW with or without combined bio-organic addition at a bio-generation plant.
AD plant selection falls into three categories, namely:
- High-rate: these categories include UASB and EGSB. They treat low-solids liquids, ideally with high carbon and properly pre-conditioned and utilise gas-assisted blanket mixing of the particulate solids. Otherwise, the process is similar but is much more restrictive in terms of permissible feedstock;
- Conventional: this includes low solids or high solids reactors – single or multi-stage. The application extends from low wastewater containing medium – high TSS and fats to the end-of-line multi-stage high solids digesters e.g. combined organic – solids and MSW treatment units. Mixing may be by gas or mechanical means;
- Alternative AD categories: this covers the larger multi-reactor applications for MSW and more recent developments including contact surface reactors and submerged media and MBR reactors.
Biogas generation, capture, storage, purification and conversion to bio-energy is the lead dependent variable in the eco-generation industry. Biogas production and quality is predictable from feedstock characterisation. It is captured to a store for purification prior to further use.
Purification is often over simplified and biological processes should be considered – possible with a secondary stage if high quality CH4 production is envisaged for normal re-use. CO2 removal is required if direct injection to natural gas mains is to be considered or if high-pressure biogas is to be considered for future filling stations.
Methane content of biogas may vary from 55-80%, again depending on feedstock. Some biogas is required to augment reactor temperature and mass and thermal balance calculations will stipulate this.
Biogas uses should be logically compared on a Sustainability Index basis i.e. Capex/Net resulting Opex of the overall system.
The engineering associated with biogas technology calls for strict application of ATEX regulations and, more specifically, with international stipulations in relation to biogas handling. An application now being considered for Ireland is the pressurised storage and transport of biogas at national gas quality levels.
Nutrient (primarily N) and digestate separation, storage, conditioning and recycle-reuse is an incoming area of importance which is initiated in association with nutrient planning frameworks. Much has yet to be done to mutual benefit of agriculture, industry and the municipal sector. The sludge assimilation capacity of usuable land areas in Ireland will, hopefully, provide enough information to formulate a national sustainable sludge strategy acceptable to all.
Conclusions, relating to the modern application of AD, are suggested as follows:
- Lead municipal and industrial corporates should re-define wastewaters, sludge solids and MSWs as co-products with potential asset value;
- The co-products – liquid and solid – from the agricultural, industrial and municipal sectors are but divisional contributions to a common co-product load. Ongoing internal research (Agro Cycle H2020) addresses this. Field applications of the principle should proceed immediately in the interest of national sustainability compliance. Earlier-proven R&D is already successfully working internationally and simply requires incentives to install in Ireland after proper feedstock characterisation;
- Mainstream AD has, in my opinion, been the foremost wastewater development over the last ten years in Ireland and across Europe. When this is applied with wastewater polishing for re-use (final effluent), it may require judicious process design, but this is proven;
- Commercial evaluation of alternatives should now define a Sustainability Index (Capex/Net reduced total Works Opex) in place of the conventional return on investment (ROI) principle. This allows system assessment based on cost audits of the total facility as distinct from the AD unit operation only;
- The predominating problem of excess solids production may be reduced from 100% to between 20-40% of the conventional estimates depending on feedstock solids and AD-based treatment facility;
- The integrated principles of nutrient recycle, humus re-use with Class B quality and local regional heating forms part of sustainability goal of the future;
- Using proper feedstock selection and mainstream AD technology in industrial applications, the bioenergy production from wastewater and solids for a typical organic industry could increase from less than 20% to greater than 70% of the industry base energy requirements.
Note to readers: This is a summary of the main paper, which also carries the relevant references.
Seamus Crickley BE, EurIng, CEng, FIEI, WEF,
Director, WEW Engineering Ltd Consulting Engineers
Crickley has specialised in best available technologies of water, wastewater and energy management over 40 years since his qualification and postgraduate studies at UCD.