Anaerobic digestion is a well-established process used in the direct treatment of sewage or the stabilisation of biosolids to convert solids to energy. Whereas in anaerobic pre-treatment the reactor will be directly fed at ambient sewage temperature, the anaerobic stabilisation of solids is often practised at mesophilic temperatures (35°C) in order to optimise the solids destruction. This article compares the energy balance of the two approaches, comparing anaerobic pre-treatment with solids digestion. Since the quantity of biogas generally produced has more energy than is required as heat to maintain the process, there is surplus energy that may be used for other purposes. It is often commercially viable to use the biogas produced in the digestion process as fuel in an internal combustion engine and to generate both electrical and thermal energy from the biogas.
In order to recover the energy in the biogas, it will be burned in an internal combustion engine. This produces work at the engine shaft, which drives a generator, and heat in both the engine and the exhaust gases. By linking the engine to a generator the work can be converted into electrical energy. The heat produced by the engine is initially removed by exhaust gases and the cooling water. As illustrated in the Sankey Diagrams, heat exchangers can recover a good proportion of this thermal energy. The low efficiency of a gas-burning engine is used to advantage in a combined heat and power unit (CHP). The heat that would otherwise be wasted is recovered and used as a source of energy for the digestion process. The overall thermal efficiency of a CHP unit is typically in the range of 80-84%.
Biogas is created by the destruction of volatile matter in the anaerobic process. Bacteria use the organic matter in the solids as substrate for energy production and growth and release methane (CH4) and carbon dioxide (CO2) in the process. The amount of biogas created and its energy content depend on a number of factors that are multiplied to yield the amount of methane – if sewage is directly digested, the typical conversion is 0.35m3 of methane/kg of COD removed. Therefore every m3 of wastewater, based on a raw COD of 600mg/l and a removal of 60% will yield a methane volume of 0.126m3. Depending on the operating conditions and sewage temperature, both the methane yield and COD removal can each be up to 10% higher, increasing the potential methane production to close to 0.17m3/m3.
With conventional primary and secondary treatment, the residual solids from both steps will provide the energy source. Based on typical values in Europe and the US, the volatile fraction of the primary solids is between 70-80% and activated sludge has a volatile content of 75-85%.
Thus, when the solids from primary and secondary treatment are converted, the typical gas production would be:
- primary settler efficiency of 50% solids would yield 150mgTSS/l at a volatile fraction of 75%,
- the biological reactor would be fed with 200mg BOD/l,
- the biomass yield would be 0.7mgTSS/mg BOD x volatile fraction of 80%,
- destruction of volatile solids in the digestor at 50%,
- biomass yield/solids destroyed at 1m3/kg VS destroyed,
- total biogas is 0.11m3/m3 of influent,
- methane content is 65%, specific CH4 production is 0.07m3/m3.
This comparison shows the direct digestion of sewage yields up to twice as much biogas as extracting the solids and then digesting them. In addition, when the solids are digested at a high rate, energy for heating has to be supplied to the anaerobic digestors, reducing the net output available for downstream.
On the other hand, one has to consider the energy need of the treatment plant, to determine how much of the power can be supplied by using the biogas. Compared to conventional primary treatment, the BOD removal efficiency of anaerobic pre-treatment is above 60% or twice as high, reducing the needs for
aeration energy downstream. Based on an overall energy need for oxidation of 1.5 kWH/kg BOD, the two treatment cases would result in the following power demands:
- anaerobic pre-treatment 300mg BOD/l x 40% remaining x 1.5 = 0.18 kwH/m3,
- primary settling 300mg BOD/l x 70% remaining x 1.5 = 0.315kwH/m3.
Typically, the methane fraction in biogas is 65% in both cases and the methane related energy content of biogas 36,000 kJ/m3. Considering a CHP engine can convert about 35% of the thermal input to electrical energy, the net electricity production/m3 of methane would be 0.35kWh. The biogas production and energy need of both cases is illustrated in the enclosed table, the net balance becomes:
- anaerobic pre-treatment has an energy surplus of 0.27kWh/m3,
- conventional treatment has an energy deficit of 0.06 kWh/m3.
It becomes clear anaerobic treatment has an enormous potential, given it will lead to energetically self-sufficient plants and a solids production of only 30% of conventional approaches. Up to now the anaerobic reactors have been used mainly in tropical countries with sewage temperatures of 20°C or higher.
One of the reasons combined heat and power conversion is not often applied on WwTWs is the complexity of the necessary infrastructure to make the system work, distracting the personnel from their main activity of treating water and solids. The following equipment needs to be installed and optimised to harness the energy in sewage:
- water separator to remove any moisture, foam or sludge that may be carried over with the gas,
- scrubber and a dehumidifier to remove H2S and moisture that is not desirable in the gas,
- gas pressure-boosting station for use by the boiler plant and gas engine driven generators,
- gas holders and waste gas burners.
The dehumidifiers will cool the biogas, thereby reducing the moisture content prior to the gas holders to a dew point of
-20°C. The scrubbers will reduce the H2S to below 600ppm and siloxanes to the levels required by the gas engines. The gas holders will store the equivalent of a few hours of gas production at peak production rate. The blowers will raise the pressure of the biogas to that required by the gas engines. The flare stack will burn the excess biogas at the maximum production rate.
The biogas can then be used as fuel for spark ignition gas engines, with the power generated by these engines driving the following:
- a CHP gas engine driven electrical power generation plant,
- the WwTW air blowers.
It could be possible to drive blowers directly with gas engines, avoiding conversion and transmission losses, and the investment and operation of the CHP engines. If a CHP plant is chosen to cover other energy needs that aeration blowers or export electricity, the installation would typically comprise:
- a generator building,
- boiler room housing the standby boiler plant, rated for the combined heating load of the digestion plant,
- building mechanical annexed housing: generator mechanical auxiliary equipment that is not skid-mounted on the respective generator sets, CHP heat recovery system heat exchangers and associated water pumps, generator set starting system equipment, engine exhaust silencers, roof-mounted, gas engine jacket water boiler and heat recovery system water header tanks, gas engine lube oil header tanks, engine gas supply interface equipment.
- an electrical annexe incorporated to the CHP generator building would house: medium voltage switch-gear room incorporating generator neutral earthing equipment, power transformers bays, low voltage switch-gear and generator control plant room, workshop and stores, externally mounted engine heat dump radiators.
In addition to producing more power and less solids with anaerobic pre-treament there is less necessity for heating as there is with solids digestors. Therefore, the heat recovery and sludge heating system can be simplified or even omitted, removing the heat recovery elements of the system, including heat exchangers and back-up dual fuel (gas or oil) boilers.
As the emissions of greenhouse gases become more and more restricted, the incentives to convert sewage to energy will increase. Up to now, most anaerobic treatment plants still flare off their biogas, often because no polishing aerobic system is yet installed. But since methane is ten times more effective than CO2 in causing climate warming, it is imperative the gas is captured and used rather than vented