Anaerobic digestionCompostingDedicated incinerationThermal drying with incineration of the refuse-derived fuel (RDF)Other treatment & disposal options for sludgeBenefits
The influent sludge will contain a variety of organic and inorganic material. Of the organic material in the sludge, only a part will be readily degradable in the anaerobic digestion process. Sugars, lipids, protein, and even some organic chemical compounds will be used as substrates for the anaerobic bacteria. Lignin, a constituent of paper, will not be degraded anaerobically, and neither will a range of synthetic chemicals that are present in domestic sewage.
Bacterial degradation of organic material. Three groups of bacteria are involved in the anaerobic digestion: the hydrolytic and fermentative bacteria, the acetogenic bacteria, and the methanogenic bacteria. The hydrolytic and fermentative bacteria will break down polymers like cellulose and proteins. Cellulose will be hydrolysed to smaller sugars. Sugars are fermented to long and short chain organic acids. The acetogenic bacteria will use the resulting compounds from the hydrolysis and fermentation, e.g. they will use long chain organic acids as a substrate and produce short chain organic and carbon dioxide. The methanogenic bacteria are the most sensitive and slow growing of the three groups of bacteria. The methanogenic bacteria use the products of the acetogenic bacteria as a substrate for methanogenesis, the production of methane. Once the methanogenic population has build up, a balance should exist between the methanogenic and acetogenic populations which is essential for both populations, and is therefore essential for the stability of the process.
Microbial requirements. In order to have a successful anaerobic digestion with quick removal of organic material, stable gas production and sufficient reduction in the number of pathogens, the bacteria must have optimal growth conditions. Most important is the control of temperature and substrate availability. Temperature optima for the anaerobic bacteria exist in the mesophilic (30-37°C) and the thermophilic (50-65°C) range. Substrate availability depends on the organic loading of the influent sludge. If the influent sludge is very thin, i.e. has got a low organic loading, the flow rate through the digester can cause wash out of the methanogenic population.
Design and operation
A lot of research has gone into finding the optimal design for an anaerobic digestion system. The solution chosen will depend on the amount of sewage sludge to be treated.
Mesophilic or thermophilic digestion. Anaerobic digestion at thermophilic temperatures has proven to be feasible, but only very few plants run at thermophilic temperatures. The thermophilic process has a greater conversion of volatile solids to gas and an increased rate of digestion, but has got a reduced process stability and a greater heat requirement. The higher risk of having problems with maintaining a stable thermophilic digestion and problems with odours from thermophilic digestion, are the reasons why digestion at mesophilic temperatures is by far the most common (ref.15).
Single-stage or two-stage digestion. In a conventional single-stage digestion system, all phases during the anaerobic degradation takes place in the same reactor, even though the microbial requirements of the different phases are not the same. In a two-stage digestion system, phase optimisation is possible. This leads to increased stability and higher biogas production due to more volatile solids being degraded (ref.3; ref.18; ref.17). However, the relatively small increase in efficiency may not justify the increased cost of two-stage digestion (ref.3). Another advantage of two-stage digestion can be utilised if the primary stage is a thermophilic stage which provides pasteurisation of the sludge. The secondary step should be mesophilic because that will eliminate problems with odours which can be a problem for thermophilic digestion (ref.19).
Requirements to influent sludge.
Dry solids content
Before feeding to the digestion tank, the raw sludge undergoes a thickening process. A thickness of 8% dry solids is desirable, and under 2.5% dry solids the performance of the digestion is reduced. A low solids concentration will lead to a short retention time and thereby a potential wash out of the methanogenic population. The thickness of the sludge is also important for its mixing properties, and it influences the volume of sludge that requires heating up to the digestion temperature (ref.5).
Inhibitory compounds
The anaerobic bacteria can be inhibited by a number of compounds, and reduced digestion efficiency can be a result of inhibitory compounds present in the sewage. Potentially inhibitory compounds include heavy metals, chlorinated organic substances, pesticides, and detergents. Due to industrial effluent control, it is unlikely that these compounds will be present in the sewage in inhibitory concentrations (ref.5; ref.26)
Digestion tank. The digester is an enclosed tank with attached heating and mixing systems. In recent years, the most common choice of design has been pre-fabricated digesters constructed of glass-coated steel panels with an external insulation layer (ref.5; ref.15). The digesters can be equipped with a floating roof for gas collection or a separate gas holder. In Germany and USA egg-shaped digesters are commonly used (ref.5). The egg-shape is ideal for providing complete mixing of the entire volume and the shape has a decreasing effect on the build-up of scum in the digester (ref.11; ref.16).
Heating. Anaerobic digestion is normally carried out at mesophilic temperatures and a heating system must be capable of maintaining the entire volume at 35°C, even at low temperatures in winter.
Maintaining the temperature at 35°C is not only important to achieve an efficient digestion process, it is also a requirement if the sludge is disposed of to agricultural land. The DoE Code of practice for Agricultural Use of Sewage Sludge (ref.10) has set requirements for different sludge treatment methods to achieve acceptable levels of pathogen reduction. (See Application of treated sludge to agricultural land.) For mesophilic anaerobic digestion the sludge for agricultural use must have been treated at least 12 days at 35°C.
Digester heating is normally supplied through, directly or indirectly, the combustion of the produced biogas. Traditionally, the heating was supplied by burning biogas in a hot water boiler and transferring the heat by use of heat exchangers. Depending on how the biogas is utilised, there can be other ways of recovering heat for the digestion. If the gas is used in dual-fuel or gas engines for power generation, it is possible to recover engine heat for digester heating. Spark-ignition engines for CHP are normally able to provide all the needed heat for digestion (ref.5; ref.15).
Mixing. The digester should be completely mixed to ensure efficient digestion.
Firstly, the mixing brings the raw influent sludge in contact with actively digesting sludge. It speeds up the digestion that the already existing bacterial population is brought into contact with new substrate. Secondly, the mixing ensures a uniform temperature throughout the digester. And thirdly, the mixing prevents accumulation of grit in the bottom of the digester and the build up of a scum layer on the top. Without a uniform environment in the digester, there would be pockets of sludge not degrading properly, potentially leading to undigested sludge leaving the digester and a decreased digestion rate.
Types of mixing systems
Energy recovery.
The production of biogas provides an easy possibility of energy recovery. The biogas is a mixture of methane (60-70%) and carbon dioxide (30-40%). Other gases like hydrogen sulphide, nitrogen, hydrogen and water vapour, are present in small amounts. The calorific value of the gas depends on its methane content, at 70% methane content it is approximately 23,380 kJ m-3 (ref.5).
The gas can be converted to energy for use in the treatment itself and excess electricity can be exported to the grid. Surplus biogas after use of gas for heating of the digester can be utilised in different ways. Some possibilities are
There are numerous ways of using the excess energy from the gas as heat or power in the previous or later steps of the sludge treatment.
The composting process
The sludge is dewatered to approximately 35% DS. This allows the sludge to be self-supporting in a pile or stack and so facilitate composting. It is then mixed with a bulking agent to dry out the blended mix. Bulking agents can be sawdust, leaves, paper and solid waste, however wood chips are the most common. It is mixed in a ratio of three parts wood chips to one part sludge to achieve 55% DS (ref.23; ref.30).
Composting systems. There are three main systems:
Aerated static pile
In-vessel
Screening. The bulking agent is removed from the mixture and reused several times. This can only be performed for agents like wood chips because they are so much bigger than the compost (ref.23).
Compost use
Most composts are too low in nutrients, especially digested sludge composts, to be classified as fertilisers. Their main use is as a soil conditioner, mulch, top dressing or as an organic base with fertiliser amendment (ref.30).
Relevant variables and environmental concerns
Sludge type
Temperature
Moisture
C/N ratio
pH level
Odour
Pathogens
Metals
Energy
The principle objective of sewage sludge incineration is to reduce the volume of sludge to be disposed of as a solid or liquid waste. It has been traditionally been used in the UK as a "last resort" at large works serving industrialised catchments, where the sludge has been significantly contaminated with heavy metals, for example, and the sludge is therefore unsuitable for application to agricultural land or results in uneconomic application rates.
Benefits of incineration
Its main advantages lie in the complete destruction of organic matter, the ash being inert and usually less than 25% of the original sludge volume. This can be particularly useful for industrial sludges containing toxic, poorly biodegradable components. The ash concentrates metals, and although rarely engineered into present designs, the opportunity to recover them by acid stripping or ion exchange. At present, such ash is considered inert and only under acidic or anaerobic ground conditions are metals likely to be leached out. There is also the possibility of energy recovery within the process.
Incineration process
Mechanical dewatering
Fluidized bed combustion
Waste heat recovery
Flue gas treatment
Energy recovery
There is the possibility of energy recovery in the form of heat, which in turn can be used in various stages throughout the process. The principle heat recovered comes from the flue gases after the combustion process. The flue gases leave the incinerator at a temperature of approximately 850°C and by passing these gases through a combustion air preheater the resulting heated air at a temperature of approximately 600°C can be blown back into the incinerator. By doing so results in no fuel requirement for combustion process, smaller furnace and a reduced volume of flue gases to be treated.
Further heat could be recovered from the combustion gases in a boiler/ thermal oil exchanger before the gases are cleaned up and released to the atmosphere via the stack. The recovered heat can be used for process purposes such as heating the sludge prior to dewatering or heating the stack gases (ref.2).
There is also the possibility of using the scrubber as an alternative to saving energy. After scrubbing the scrubber water is at a temperature of around 70°C and this could be used in the form as hot water to heat water from say an apartment block in the immediate area of the plant (ref.22).
The thermal drying process
The process described here is for an end product of 90-95% DS. It can however be used to produce a product with a lower percentage of dry solids as for example in the case of dedicated incineration. The reason why a product of 90-95% DS is produced is that it has low odour, it has good handling characteristics with minimal dust content, and it is pathogen free. In addition transport and storage costs are reduced because of it's minimal volume (ref.1; ref.24).
Dewatered sludge cake. The sludge is first dewatered from 6% DS to about 25% DS before it is thermally dried. Dewatering is practiced to reduce the volume of the sludge to facilitate the drying process by making the sludge more handleable and reducing transport costs. There are three categories of mechanical dewatering machines used for dewatering sludge: Centrifuges, filter belt presses and filter plate/membrane presses (ref.15).
Thermal drying. There are three types of dryers:
Emission management systems. All types of dryers produce vapour which can create an odour nuisance. Heat recovery can take place from boiler exhaust gases and the vapour stream depending on the type of emission management system used. There are three types of systems:
Backmixing. This is performed to avoid the pasty phase that the sludge goes through while drying. The dried sludge is returned to the start of the process and mixed with the incoming sludge cake to increase the dry solids content up to 60-65% and so ease handling of the sludge. Not all types of dryers need the sludge to be backmixed, however the ones that do are to be found in all categories of dryer (ref.24).
Cyclone separators. Most direct and combined system dryers need the dried sludge to be separated from the vapour after it has passed through the dryer. This is done by a cyclone which uses centrifugal force to keep the dried sludge against the wall of the separator as it falls to the bottom for collection, while at the same time the vapour rises and leaves through the top of the separator (ref.23).
Pelletisation. In some cases the actual end product has a bulk density as low as 260 kg/m3 which can cause plant operational and handling problems. It is therefore necessary to increase the bulk density up to at least 800 kg/m3. This is done by compressing the material in a die to produce a consistent and manageable product (ref.4).
Product use
Dried sludge has a number of uses such as in agriculture, horticulture, forestry and land reclamation as a soil conditioner, mulch, top dressing or as a of fertiliser. In addition it can be used as a fuel for coal fired power stations. The pellets make up less than 1% of the coal feed, consequently it has a negligible effect on the atmospheric emissions and ash content from the power station (ref.1; ref.4).
Relevant variables and environmental concerns
Sludge type
Temperature
Pressure
Odour
Noise
Pathogens
Metals
Thermophilic aerobic digestion, TAD, is a process by which the sludge is stabilised and pasteurised. Thermophilic aerobic digestion occurs when the sludge is aerated and mixed. Heat will be produced as the microbial degradation of the organic matter occurs. According to the DoE Code of Practice (ref.10), sufficient stabilization and pasteurization is achieved with a mean retention time of 7 days, where the temperature has been held at minimum 55°C for at least 4 hours. A problem is that TAD is a highly energy requiring process. TAD can be used as a primary step before anaerobic digestion in a two-stage process (ref.5; ref.15).
Lime stabilization is a process where the sludge is mixed with lime to raise the pH to greater than 12. The high pH and a rise in temperature reduces pathogens and reduces offensive odours. The DoE Code of Practice (ref.10) requires that the pH is minimum 12.0 for two hours for sufficient treatment (ref.5; ref.15).
N-Viro Soil is a process where dewatered sludge is mixed with an alkaline additive. The increase in pH and temperature stabilizes and pasteurizes the sludge. After drying and storage and turning of windrows the result is a soil-like product. N-Viro Soil can be used as an agricultural lime substitute and a low level fertiliser, or as a soil substitute for land reclamation (ref.5; ref.6).
Application of raw sludge to agricultural land is an option as long as the sludge is immediately incorporated in the soil (ref.32).
This way of recycling sludge can be problematic due to offensive odours and a potential health risk due to pathogens in the raw sludge.
Application of treated sludge to agricultural land is considered to be the best practicable environmental option (BPEO) for most sewage sludge (Department of the Environment, 1995). Treatment of the sludge should comply with the guidelines set by the DoE Code of Practice for the Agricultural use of Sewage Sludge (ref.10). Care must be taken that applying the sludge will not pose any health or environmental hazard due to it's content of pathogens and heavy metals. Furthermore, the sludge should be applied according to the fertiliser requirements of the crops grown on the land to prevent leaching of the nutrients, especially nitrogen.
Pathogen reduction
Heavy metals
Sludge products
Disposal of sludge to landfills is a possibility, but it should be considered to be the last solution according to "the waste hierarchy": recycling of or energy recovery from waste are preferable to disposal. However, energy can be recovered from landfilled sludge if landfill gas is utilised.
Disposal of ashes to landfills. Incinerator ashes are classed as industrial wastes and the landfill must have a site licence that includes this type of waste.
Egg-shaped digesters have got ideal mixing properties.
Different types of systems are used, either mechanical mixing systems or gas mixing systems. Most modern plants use gas recirculation systems where digester gas is recirculated to diffuse through the sludge. Mechanical mixing systems are cheaper, but because of the very high cost of taking a digester out of use in case of maintenance or repair, the mechanical mixing systems are not cheaper in use than gas mixing systems (ref.5; ref.15). A new mixing system, draft-tube mixers, are energy efficient, and their use is recommended especially for egg-shaped digesters (ref.31).
CHP production has existed for years at larger sewage works, traditionally using dual-fuel engines with supply of diesel. Smaller works have mainly used spark-ignition engines. Studies of the best practicable environmental option (BPEO) for use of sewage sludge have ranked CHP generation very high (ref.15).
Composting
- works by mixing the sludge with a bulking agent to make sure that the mixture can be aerated for an accelerated aerobic degradation process. The problems are that much energy is required to aerate the composting material, and the end product, the compost, must be disposed of to suitable land and it is not attractive to farmers due to a low fertiliser value.
Traditional wind-row
The mixed sludge is piled up in long rows, 1/2 m high and 3/4 m wide, in- or outdoors. Regular turning is necessary to oxygenate the entire material.
Same as above except that oxygenation is performed by forcing air through the mixture from perforated pipes lying underneath. To reduce odours a stabilized compost cover is placed on the piles.
The mixture here is placed in a reactor, there are two types of reactors:
It can be raw or digested sludge. Digested sludge use helps to produce a fully stabilized compost (ref.23).
This should be 40-60°C. Too low and the pathogens survive, too high and the compost micro-organisms necessary for decomposition may be inhibited (ref.30).
40-50%, this gives sufficient air spaces for an aerobic environment. Too high and anaerobic conditions occur (ref.30).
Should be between 20 and 30. Too high or too low and the process is inhibited. The addition of the bulking agent increases the ratio to the necessary level (ref.30).
Operational range of 6.5 to 9.5, which is greater than that for anaerobic digestion (ref.23; ref.30).
Not a problem in well aerated or in-vessel composting. Compounds causing it can be removed and converted (ref.30).
Fully stabilized compost should occur with complete degradation, 30 days approximately, however 100% sterilization is hard to verify (ref.30).
May be to high in urban sludge, however pretreatment with bauxite refined residues (red mud), especially digested sludge, may allow for it's use (ref.29).
Composting is an energy intensive process, not yielding a product or by-product suitable for electricity generation (ref.23).
Dedicated incineration
- works by dewatering the sludge to a solid content of about 25%, using either centrifuges or filter presses. The sludge cakes are then fed into a fluidized bed incinerator and incinerated at a temperature of 850°C. Energy can be recovered through a number of heat exchangers used to dissipate heat from the incineration to other heat requiring steps in the process. One of the problems is that the gases released into the atmosphere are perceived as being potentially hazardous. Planning permissions of such plants could be a limiting factor.
The first stage in the incineration process is the requirement to dewater the sludge i.e. to reduce the water content. This allows for easier combustion of the sludge and less energy requirements for the incinerator to deal with this excess water. Incoming sludge usually contains around 6% dry solids content (d.s) and 94% water content. For incineration purposes you have to dewater the sludge to produce a cake of around 28% d.s. Since combustion plant performs best in a continuos mode of operation, whatever dewatering plant is ought to have the features of continuos operation and control. In principle this will favour the selection of centrifuges or filter belt presses.
There are two main types of furnace used for sewage sludge incineration, these being multiple hearth and fluidized bed. Each have their advantages and their disadvantages, but the economic achievement of a high degree of combustion efficiency and the avoidance of odour nuisances, demands the use of fluidized bed technology. Furnace temperatures are normally controlled within the range of 800 to 900°C: at lower temperatures, incomplete combustion and odour formation becomes apparent, whilst at higher temperatures problems can be caused by ash softening (ref.15).
The flue gases which result from the combustion process are at a very high temperature and some of this heat can be recovered by passing it through a waste heat recovery unit which is discussed later on in Energy Recovery.
To meet current and future requirements an abatement system comprising two stages of gas treatment is needed. A fairly typical treatment train, based on a survey of European operational plants and proposed retrofits, would comprise: electrostatic precipitation for particulate control and wet scrubbing for acid gas control, principally, but also for control of particulates and substances of intermediate to high volatility such as cadmium, mercury and some organic compounds (ref.15). Electrostatic precipitators can have a particulate removal efficiency as high as 99% and with the use of wet scrubbing, whereby removal of 99% of HCl and 90%+ of HF can be achieved and if an alkaline scrubber is used then 80% + of SO2 can also be removed, then we can have a very efficient form of flue gas cleaning (ref.13). The ash which is removed is usually taken away and disposed of to a landfill and the scrubber liquid is treated and drained to the sewer. The resultant gas after cleaning is usually preheated to suppress the formation of a visual plume through the stack or chimney.
Sludge incineration plant.
Thermal drying
- with incineration of the refuse-derived fuel (RDF) works by dewatering the sludge to a solids content of 25-32% d.s. using centrifuges or filter presses. The sludge cakes are then thermally dried to 90-95% d.s. This fuel can be used in a coal fired power station where it is mixed with coal to a mixture of 99% coal and 1% RDF. Depending on the type of sludge, the calorific value of this RDF can be similar to brown coal. The main problem in this process is odours from the thermal drying.
Primary, secondary and digested sludge have slightly different drying characteristics and produce products with different properties. For instance, dried primary sludge will have a higher organic and nitrogen content than dried digested sludge but a lower bulk density. Dried primary sludge has a higher calorific value than dried digested sludge, and dried secondary sludge has a still higher value, making it the most suitable sludge for power generation. Other reasons why thermal drying of digested sludge is problematic, is that process time and costs are increased (ref.1; ref.4; ref.24).
Direct dryers use gases as the heating medium at temperatures of between 300 to 600°C depending on the type of dryer. Indirect dryers use steam at 135-215°C or thermal oil at temperatures of 200-250°C although temperatures as low as 105°C are being used. The greater the temperature of the dryers or the other equipment in the plant the more chance there is of the dried sewage sludge. In powder form the sludge will ignite at approximately 380-400°C, however weak exothermic reactions have taken place at temperatures as low as 100°C where the air content is between 25 to 125 g/m3. There is therefore two fire preventative measures that can be implemented, the operation of equipment in oxygen concentrations below 9-12% v/v or in temperatures well below 100°C (ref.1; ref.23; ref.15).
It is the usual practice to operate all systems under a negative pressure, this was found to prevent the escape of dust and vapour to the environment during operation (ref.1; ref.15).
This is probably the most important concern of the local planning authority who are responsible for controlling thermal drying plants. At least one plant was forced to cease operation in the 1960's due to complaints from the local population about odour emissions. Odour, however, is not a problem when semi-closed and closed-loop emission management systems are used as the odour producing trace organics and other combustibles contained in the vapour are passed through the boiler for thermal destruction before emission to the atmosphere. In addition a high dry solids percentage end product ensures low odour after the sludge has left the plant (ref.24; REF.7; ref.15).
This does not appear to be problem if the plant is appropriately designed for it's local. The only real noise nuisance will be caused during the construction of the plant (ref.15).
The percentage of pathogens eliminated is dependent on the drying temperature and the residence time of the sludge in the dryer which could be anything from seconds in the case of some direct dryers to well over half an hour. There is nevertheless a substantial kill of pathogens and viruses at temperatures at less than 100°C so a fully pasteurised product is not hard to achieve under the right operating conditions (ref.1; ref.15).
Heavy metals are not destroyed by the thermal drying process it is therefore a concern where the dried product is used in agriculture and similar product outlets, however heavy metal concentrations in UK sludge usually accord with EEC permissible levels. Other potentially hazardous substances that are unaffected by the drying process are plastics, dioxins and PCB's (ref.23; ref.28).
Other treatment and disposal options for sludge
Apart from the above mentioned technologies, there are a number of other ways in which sewage sludge can be treated or disposed of. These methods do not provide energy recovery from the sludge.
Sludge treatment
Sludge disposal
Stabilised (treated) sludge has no longer any offensive odours.
The treatment processes which in the Code of Practice are considered appropriate to ensure suffient stabilization and reduction of pathogens is shown in table 1.
Process Description
Sludge pasteurization Minimum 30 min at 70°C or minimum of 4 h at 55°C, followed by mesophilic anaerobic digestion.
Mesophilic anaerobic digestion Mean retention period of at least 12 days primary digestion at 35°C or at least 20 days primary digestion at 25°C, followed by a secondary stage with a mean retention period of at least 14 days.
Thermophilic aerobic digestion Mean retention period of at least 7 days digestion. All sludge to be subject to a minimum of 55°C for a period of 4h.
Composting Must be maintened at 40°C for at least 5 days and for 4h during this period at a minimum of 55°C within the body of the pile, followed by a period of maturation to ensure complete compost reduction.
Lime stabilization of liquid sludge Addition of lime to raise the pH to 12.0 and sufficient to ensure that the pH is not less than 12.0 for a minimum period of 2h.
The addition of heavy metals from sludge to soil is legally restricted. The maximum rates of application are considered on a 10 year basis. Over a 10 year period, an average addition to the soil must not exceed the values given in table 2.
Heavy metal Zn Cu Ni Cd Pb Hg Cr kg/(ha yr) 15 7.5 3 0.15 15 0.1 15
The application of sludge can occur as liquid sludge, sludge cakes (25% dry solids), or dry sludge pellets (95% dry solids). Nutrients are lost in the dewatering and drying processes. Even though nutrients are lost and energy used in the drying process, dry pellets have got advantages:
- they are easy to spread for the farmer who can use standard fertiliser spreading equipment
- they can be stored easily by the farmer
- they have got a non-offensive appearance
and for these reasons are sludge pellets are more attractive for farmers than other sludge products. A survey of farmer's willingness in the Strathclyde region to use different sludge products showed that more farmers (56%) were willing to use sludge pellets than liquid (40%) or caked (28%) sludge (ref.31).