• Energy-from-Waste technologies – Biological Treatment: Anaerobic Digestion

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    • Post Author

      Philip Sharman

      IFRF Director

Over the last four months, we have shared a number of Monday Night Mail pieces on the subject of ‘energy-from-waste’ (EfW) which we thought might be of interest to IFRF members.  In the first of these, we discussed a report from US consultants Grand View Research (GVR), published in November, examining the size of this market and growth trends out to the middle of the next decade.  On 5th March, we started to unpack the EfW topic from a technology and ‘R&D needs’ angle, looking at the main ‘thermal treatment’ process used in current EfW plants, namely incineration.  Last month , we examined so-called ‘advanced thermal treatment’ (ATT)processes based on pyrolysis and gasification technologies.  Now, in the fourth in this series, we take a look at the final process route option – ‘biological treatment’ – which includes aerobic processes as well as anaerobic digestion (AD).  As IFRF members are likely to be less interested in aerobic processes (i.e. composting) – keen gardeners excepted – we will focus on AD processes that yield combustible gases (i.e. ‘biogas’ and ‘biomethane’)!

The consultancy report from GVR referred to above (here) indicated that while thermal treatment techniques – primarily incineration but also increasingly ATT techniques too – accounted for around 80% of the $25 billion EfW market in 2015, significant growth in EfW plants based on biological treatment is expected over the period from 2016 to 2024, accounting for over 21% of the market by the end of that period:  This is mainly due to its potential in developing markets.

This growth is certainly apparent in Europe where, according to the European Biogas Association’s 2017 Statistical Report, 2016 saw the number of biogas plants reach 17,662, adding to a long-term trend that has seen the number of such plants in Europe almost tripling between 2009 and 2016, with the UK, Belgium and the Netherlands particularly active.  The total installed electric capacity (IEC) of these plants reached almost 10GW in 2016, up 9% in 2016 alone. Biomethane production (from upgrading biogas) was up 40% from 2015, to 17,264GWh, with Germany, France and Sweden contributing strongly to this increase.

Biological treatment technologies for EfW

Firstly, it is important to note that the biological treatment of wastes – predominantly AD but also aerobic techniques such as the bio-drying and bio-stabilisation (partial composting) of the whole waste stream and in-vessel composing (IVC) – is often used as part of the pre-treatment of residuaxl waste within a ‘municipal solid waste (MSW) management strategy’ (e.g. in an integrated EfW facility).  Such pre-treatment approaches are usually referred to as ‘mechanical biological treatment’ (MBT) and are often utilised in incineration- and ATT-based EfW facilities (this was mentioned but not elaborated in the previous MNM articles).

MBT pre-treatment facilities for residual waste may be configured in a variety of ways to achieve the required performance for recycling, recovery and diversion of biodegradable municipal waste (BMW).  Such configurations generally include the following stages:  a waste preparation stage (before biological treatment and/or waste separation) – using equipment such as bag splittersrotating drums(dry or wet, with/without knives), shredders and mills (hammer mills, ball mills, etc.); a waste separation stage (before or after biological treatment) to sort the waste into various fractions using mechanical means – using manual separation or separation equipment such as trommelsscreensmagnetic separatorseddy current separatorswet separatorsair classifiersballistic separators and optical separators; and a biological treatment stage (see below).

Further information on MBT pre-treatment of MSW can be found here.

In addition to their role within MBT pre-treatment processes for MSW, biological treatment processes are mature technologies that are widely used in various industrial and commercial operations (notably in food and drink manufacturing, water and waste water treatment, etc.) and in the agricultural sector (both in centralised facilities and at individual farms) to treat a wide variety of waste streams including BMW (see above), source-segregated wastes (e.g. food waste), green garden waste, sewage sludge, agricultural waste, animal waste, etc.  Increasingly, such applications are moving towards EfW operation due to the obvious benefits in terms of energy production and sales, and it is in that context that we now look at the specific biological treatment technologies.

Biological treatment is based on the decomposition of biodegradable wastes by living microbes (bacteria and fungi) which use the waste/feedstock as a food source for their growth and proliferation.  The treatment occurs either in the presence of oxygen (aerobically) or in the absence of oxygen (anaerobically).  As has already been mentioned in the context of MBT pre-treatment, aerobic treatment routes include bio-drying and bio-stabilisation, along with in-vessel composting (IVC).  IVC and AD are often referred to as ‘advanced biological treatment’ (ABT) processes as they are tightly-controlled treatment routes.  In the context of these articles on EfW, it is AD that is of particular interest as it produces usable, combustible gases.

Waste feedstock to an AD unit is mixed and macerated with a large proportion of recirculated process effluent and/or fresh water to give it the required moisture and flow properties.  AD processes are generally categorised as either ‘wet’ (used for materials with a moisture content more than ~85% and that are readily converted into liquid, e.g. food waste) or ‘dry’ (used for materials with a moisture content less than ~80%, and with more ‘structure’, e.g. green garden waste and energy crops).  In wet AD systems, the digestion process takes place in sealed vertical tanks (‘digestors’) that are usually continuously mixed to maximise contact between microbes and waste.  Mixing is achieved using mechanical stirring devices or by recirculating biogas or waste through the tank.  Some wet AD systems require de-packaging equipment to remove packaging contaminants in some source-segregated waste streams, or decontaminate mixed residual MSW by gravimetric separation.  Dry AD systems, on the other hand, utilise plug flow ‘reactors’ either configured vertically (using gravity to move the material down through the reactors) or horizontally (using augers or baffles).  Dry AD systems can generally tolerate higher levels of physical contaminants than wet systems.

During the AD process, biodegradable material is converted into methane, carbon dioxide (CO2) – together referred to as ‘biogas’ – and water through fermentation in the absence of oxygen, leaving a partially-stabilised, wet, organic mixture.  As anaerobic processes require less energy input than aerobic composting and create much lower amounts of biologically-produced heat, additional heat may be required to maintain optimised process temperatures:  However, since the biogas produced contains more energy than the process requires, AD is a net producer of energy.  AD generally takes around 3-6 weeks, depending on the ease and degree to which materials are converted into biogas and the technology used (e.g. high lignin content – ‘woody’ – waste will require longer residence times to achieve the desired biogas production).

AD technologies can be operated at moderate – ‘mesophilic’ – temperatures (30-40°C) or at higher – ‘thermophilic’ – temperatures (50-60°C).  Dry AD processes lend themselves to thermophilic operation due to the higher solids content (often 20-45%) and greater biological heat production, although can operate in either temperature range.  Wet AD can be conducted in either temperature range, but are most commonly mesophilic.  In mesophilic operations, a pasteurisation step is required to heat the material either before or after digestion in order to achieve sanitisation.

In addition to categorising AD processes based on the moisture content of the waste (wet, dry) and the temperature of the digestion process (mesophilic, thermophilic), they are also categorised on the basis of the number of process steps (i.e. tanks/reactors):  ‘single-step’ (one digestor/reactor stage), or ‘multiple-step’ (using a number of digestors/reactors to optimise different stages of the process).  Multiple-step AD plants often utilise a separate hydrolysis stage (aerobic or anaerobic) to breakdown complex organic material into soluble compounds, followed by a high-rate biogas production stage.  Pumps are used to transfer material between the stages.

In the energy recovery stage, the biogas is collected and stored in large vessels prior to use on- or off-site.  It is primarily composed of methane (typically 50-75%) and carbon dioxide, with smaller quantities of other gases including hydrogen sulphide (H2S), and is water saturated (i.e. 100% humidity).  The amount produced varies depending on process design (e.g. retention times, operating temperatures, etc.) and the volatile solids (organic matter) content of the feedstock.

Some or all of the biogas is usually combusted in a boiler to produce hot water and/or steam, or, increasingly, in a CHP generator unit to produce heat and electric power.  In either case, once the heat and power needs of the AD plant have been met, surplus heat can be used locally (e.g. by neighbouring commercial or industrial operations or via a district heating scheme), and surplus electricity dispatched to the local electricity distribution network.

Biogas electricity production can range from 75 to 225kWh per tonne of waste, depending on the feedstock composition, biogas production rates and the electricity generation equipment used.  The resulting electricity is generally considered as a renewable energy source.

In most simple energy recovery schemes, only minor pre-treatment of the biogas is required:  For use in a boiler, only minimum treatment and compression is needed, as boilers are much less sensitive to contaminants such as H2S and moisture, and can operate at much lower input gas pressures.  For on-site electricity generation, a generator similar to landfill gas application can be used, and again such equipment is less sensitive to H2S and moisture, although gas compression equipment may be required to boost the gas pressure to that required by the generator set.

Increasingly, there is interest in upgrading biogas into ‘biomethane’ and injecting it into the gas grid as a natural gas substitute, or for use as transportation or engine fuel.

For these higher specification applications, or when more sophisticated CHP or power generation equipment is used (e.g. turbines), the biogas will require more pre-treatment to upgrade its quality – H2S and moisture removal to reduce corrosion of equipment, pressurisation, and CO2 removal to increase the calorific value of the gas.

The remaining material passes to the digestate handling stage.  Referred to as ‘digestate’, it consists of a partially-stabilised wet solid or liquid suspension of non-biodegradable materials, recalcitrant organics, microbes and microbial remains, and decomposition by-products.  Stored in tanks, the digestate can be de-watered by mechanical pressing into its solid and liquid fractions – ‘de-watered digestate’ and ‘liquor’.  The de-watered digestate may be used directly on land as a soil amendment product (provided it meets regulatory standards and environmental permitting requirements), or aerobically treated to produce a compost (if from a source-segregated material) or ‘compost-like output’ – CLO (if from mechanically-separated material).  Some of the liquor may be recycled within the AD process to ‘wet’ the incoming feedstock, used as a liquid fertilizer, or used to maintain moisture levels during the aerobic treatment of the de-watered digestate.

As has been described in previous MNM articles addressing thermal processes used in EfW applications, emissions control is a key aspect to be managed.  The primary emissions from AD-based EfW plants are emissions to air, potential discharges to water by leachate and land impacts from the application of soil conditioners.

Air emission concerns centre on the health effects of bio-aerosols associated with the processing of large amounts of organic materials.  Bio-aerosols are complex mixtures of micro-organisms transported in the air, some of which can cause health problems (notably Aspergillus Fumigatus and some other fungal spores and bacteria).  There is no evidence of increased risk of cancer or asthma resulting from AD operations, and siting potentially emitting process steps within enclosed buildings and locating AD plants more than 250m from communities are sound precautionary strategies.  Dust and odour emissions are minimised through combustion of the produced biogas, and where this isn’t the case, biofiltration has proved an effective mitigation option.

Biological treatment (aerobic and anaerobic) of waste also offers the opportunity for nitrogen, phosphorus, potassium and trace metal nutrients to be retained in the agricultural/horticultural cycle – in contrast to thermal treatment approaches.

Special care needs to be taken when applying biological treatment to animal by-products or catering wastes.  Strict regulations (e.g. the EU Animal By-product Regulations) are enforced in order to maintain hygiene standards.

R&D challenges for AD-based EfW plant

As was stated in the MNM piece of 5th February, the main R&D challenges to address for EfW systems in the future generally relate to improving the health and environmental aspects, reducing the costs of installation of EfW systems and improving the efficiency of waste conversion (i.e. the energy recovery stage).

For AD specifically, R&D (and in some cases, demonstration) activities are focussing on:

  • Component-level improvements to reduce costs, utilise a wider range of feedstocks, use digestate residues, monitor process performance and optimise bacterial strains;
  • Development of improved AD pre-treatment, digestion and gas-upgrading components that are robust to a variety of feedstocks;
  • Improved catalysts and clean-up technologies to enable cheaper, more reliable production at smaller scales;
  • Development of advanced boilers and operating systems that are robust to a variety of feedstocks (e.g. organic Rankine cycle);
  • Establish combustion facilities that are compatible with carbon capture and storage (CCS);
  • Increased deployment of AD-derived heat through improvements in construction methods, production and distribution control systems, and heat delivery methods;
  • Improved electricity generation systems with higher generating efficiencies;
  • Improved gas upgrading and injection system, e.g. using pressure swing adsorption, membrane separation and cryogenic separation; and
  • Large-scale demonstration of a fully integrated AD-to-biomethane plant.

For further information about EfW based on biological treatment see here.