Principles and potential of the anaerobic digestion of waste-activated sludge
Introduction
When treating municipal wastewater, the disposal of sludge is a problem of growing importance, representing up to 50% of the current operating costs of a wastewater treatment plant (WWTP) [1]. Municipal WWTPs generate sludge as a by-product of the physical, chemical and biological processes used during treatment. Current daily amounts, expressed as dry solids (DS) range from 60 to 90 g DS per population equivalent (p.e.), i.e. nearly 10 million tons of dry sludge per year for the EU.
This sludge must undergo some treatment in order to reduce its associated volumes, to improve its character and to reduce the associated health problems and hindrance. This treatment will hence (i) firstly reduce the water content of the raw sludge, (ii) transform the highly putrescible organic matter into a relatively stable or inert organic and inorganic residue, and (iii) finally condition the residue to meet disposal acceptance regulation. Since land application is difficult due to stringent regulations concerning the tolerated composition [2], [3], [4], (co-)incineration is gaining increasing interest where permits can be obtained [5].
The water purification part of a WWTP commonly comprises a pre-treatment to remove about 50–60% of the suspended solids and 30–40% of the BOD [6], [7]. The settled primary sludge contains mainly water (between 97% and 99%) and separates mostly organic matter that is highly putrescible.
The pre-treatment is followed by a biological step, where aerobic micro-organisms remove the remaining (or nearly total) BOD and suspended solids. Nitrogen (N) and phosphorus (P) are commonly removed simultaneously, although N is more usually and easily targeted first. A secondary clarifier produces the dischargeable effluent as overflow and a bottom sludge (98–99% water), partly recycled to the biology to maintain the concentration of the micro-organisms at the required level, and partly evacuated to the sludge treatment units of the WWTP. If a pre-treatment is present, primary and secondary sludge are generally combined and thickened to undergo further treatment.
This further treatment can be a combination of various steps, as reviewed in Table 1. Anaerobic digestion (AD) is an important step in most of the treatment routes.
All routes start with raw sludge (primary and secondary) produced at 1–2 wt% DS. The mineral part of the DS (MDS) is between 30 wt% and 45 wt%.
A first step is its thickening by gravity, flotation or belt filtration. In doing so, the amount of sludge can be reduced to as little as a third of its initial volume. The separated water is recycled to the influent of the WWTP. Once this has been accomplished, the sludge is subject to some form of biochemical stabilisation, with AD playing an important role for its abilities to further transform organic matter into biogas (60–70 vol% of methane, CH4), thereby also reducing the amount of final sludge solids for disposal is also reduced, destroying most of the pathogens present in the sludge, and limiting possible odour problems associated with residual putrescible matter.
For these reasons, anaerobic sludge digestion optimises WWTP costs and is considered a major and essential part of a modern WWTP. The potential of using the biogas as energy source is widely recognised. Biogas is currently produced mostly by digestion of sewage treatment sludge, with minor contributions from fermentation or gasification of solid waste or of lignocellulosic material (processes currently being further developed). It is considered an important future contributor to the energy supply of Europe, although upgrading is needed.
The annual potential of biogas production in Europe is estimated in excess of 200 billions m3.
AD of sludge uses airtight tanks. Essentially all organic material can be digested, except for stable woody materials since the anaerobic micro-organisms are unable to degrade lignin. The biogas which is formed has a high calorific value and is considered as a renewable energy source. Clearly, it is beneficial to produce as much biogas as possible. Despite these advantages of AD, some limitations are inevitable, e.g. (i) only a partial decomposition of the organic fraction, (ii) the rather slow reaction rate and associated large volumes and high costs of the digesters, (iii) the vulnerability of the process to various inhibitors, (iv) the rather poor supernatant quality produced, (v) the presence of other biogas constituents such as carbon dioxide (CO2), hydrogen sulphide (H2S) and excess moisture, (vi) the possible presence of volatile siloxanes in the biogas that can cause serious damage in the energy users (generator, boiler) due to the formation of microcrystalline silica, and (vii) the increased concentration of heavy metals and various industrial “organics” in the residual sludge due to the significant reduction of the organic fraction during digestion, leaving the mineral and non-degradable fraction untouched.
A process flowchart of the sludge-processing steps is shown in Fig. 1.
The present paper will attempt to extensively review the principles of AD of sewage sludge, the process parameters and their interaction, the design methods, the biogas utilisation, the possible problems and potential pro-active measures, and the recent developments to reduce the impact of the difficulties described above.
Section 2 will review the basic principles and parameters of the AD process, including the process description, the types of anaerobic digesters (standard rate, high-rate, two-stage, mesophilic, thermophilic), the current empirical design methods, the common operating parameters and the resultant biogas yields.
Modelling and monitoring the AD process are dealt with in Section 3: models can tentatively be divided into either simple steady-state models or complex dynamic simulation models. When required system performance criteria are defined, steady-state models predict the operating parameters and lead to a system design with reasonable accuracy. These approximate design and operating parameters can then be used as input to the more complex simulation models to investigate the dynamic behaviour of the system and fine-tune the design and operating parameters in real-time.
Having studied the dominant parameters, Section 4 will focus on the operational vulnerability of digestion. The microbiology of the AD is complex and delicate, involving several bacterial groups, each of them having their own optimum working conditions. They are sensitive to several process parameters such as pH, alkalinity, concentration of free ammonia, hydrogen, volatile fatty acids (VFA), etc. These parameters can be inhibiting factors to some or all bacterial groups, and modern approaches include these inhibiting effects in modelling, in investigating the behaviour of the system and in controlling the process.
Section 5 will describe novel methods to accelerate the digestion through enhancing the rate-limiting hydrolysis. Various pre-treatments have recently been studied and include mechanical, thermal, chemical and biological interventions. All pre-treatments result in a lysis or disintegration of sludge cells, thus releasing and solubilising intracellular material into the water phase and transforming refractory organic material into biodegradable species, therefore making more material readily available for micro-organisms. It will be shown that these pre-treatments enhance the biogas generation. Since the degradation rate is moreover accelerated, the dimensions of the digesters can be reduced for a given load, thus reducing the capital requirements.
Section 6 will focus on the possible techniques to upgrade the biogas formed by removing CO2, H2S and excess moisture. A special attention will be paid to the problems associated with siloxanes (SX), including their origin and behaviour in sludge, and the techniques to either reduce their concentration in sludge by preventive actions such as peroxidation, or to eliminate the SX from the biogas by adsorption or other techniques.
Section 7 will guide the reader to extensive publications concerning the operation, control, maintenance and troubleshooting of AD plants.
Section snippets
Principles
The AD of organic material basically follows; hydrolysis, acidogenesis, acetogenesis and methanogenesis as shown in Fig. 2. The biological aspects of AD are dealt with in specialised literature [8], [9], [10], [11].
AD is a complex process which requires strict anaerobic conditions (oxidation reduction potential (ORP)<−200 mV) to proceed, and depends on the coordinated activity of a complex microbial association to transform organic material into mostly CO2 and methane (CH4). Despite the
Inhibition
Inhibiting compounds are either already present in the digester substrate or are generated during digestion.
Perspectives
As produced by digestion, biogas is a clean and environmentally friendly fuel, although it contains only about 55–65% of CH4. Other constituents include 30–40% of CO2, fractions of water vapour, traces of H2S and H2, and possibly other contaminants (e.g. siloxanes).
Without further treatment, it can only be used at the place of production. There is a great need to increase the energy content of the biogas, thus making it transportable over larger distances if economically and energy sensible.
Operation, maintenance and troubleshooting of digesters
ADs operate in a stable way if solids levels and the alkalinity/acid ratio are controlled. They have large inertia, even at 12 days.
The routine operation needs adequate maintenance and repairs, cleaning and start-up/shutdown procedures.
A very detailed account for operations, troubleshooting and control has been published by EPA [206] and summarised by Qasim [7]. The reader is referred to this extensive summary, which deals with: (i) digester start-up, involving the start-up sequence and actions
Conclusions and recommendations
AD is a complex process which requires strict anaerobic conditions to proceed, and depends on the coordinated activity of a complex microbial association to transform organic material into mostly carbon dioxide (CO2) and methane (CH4). Despite the occurrence of successive steps, hydrolysis is generally considered as rate-limiting. Within the anaerobic environment, various important parameters affect the rates of the different steps of the digestion process, i.e. pH and alkalinity, temperature,
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