Sludge from wastewater treatment plant

Sewage sludge is the main waste product of wastewater treatment process. The cleaning process is carried out in a wastewater treatment plant (WWTP), which produces water on the outlet with significantly reduced levels of pollutants. Undesirable components in the water are concentrated into sludge. Sludge from the treatment plant consists of a suspension of solid, organic and inorganic substances in water. When treating waste water the so-called primary sludge is produced, which settles in the primary sedimentation tanks and in the process of biological treatment of waste water, excess sludge (secondary) is formed, which is deposited in secondary clarifiers. Excess sludge mixed with the primary sludge is referred to as the mixed sludge. Tab. 3 shows the average quantity of treated sludge at selected WWTPs.

Sludge stabilization

Mixed sludge is hygienically defective because it contains pathogenic micro-organisms (e.g. salmonella and faecal streptococci) and a considerable quantity of organic substances (approximately 70% of the total dry weight of the sludge), which form the odour. For these reasons it is necessary to stabilize the sludge.

In general, the predominant method of sludge treatment is their anaerobic stabilization - methanization in which a mixed culture of microorganisms degrades the biodegradable organic matter without access to air. During the sludge methanization the organic solids content decreases by 45-65%. The result of the stabilization is the supernatant/sludge gas, resulting biomass of anaerobic micro-organisms and non-degradable residual sludge, which is, from a hygienic and sensoric point of view, harmless to the environment – stabilized.

Anaerobic sludge stabilization occurs in digestion tanks (30). The tanks are equipped with a stirring mechanism to ensure effective contact of the active biomass with the sludge, and with an outlet for sewage gas and stabilized sludge. Digestion tanks are in continuous 24-hour operation. Attendance of the technology of sludge stabilization consists mainly of securing the maintenance of a stable temperature of 40 °C (mesophilic digestion method), regular dosage of mixed sludge in the digestion tanks and chemical-technological control of the ongoing process of sludge processing.

Stabilized sludge processing

For better handling of stabilized sludge, it is necessary to remove from the sludge as much water as possible. Stabilized sludge is separated from the sludge water in storage tanks (31). Sludge water is drained off by pipeworks to the WWTP inflow and thus it passes through the complete cleaning process. Stabilized sludge from storage tank or from the additional gravitational thickening, with the sludge dry matter content in the range 2.5-4.0%, is further mechanically dewatered through a decanting centrifuge (38). The result of dewatering is the separating of the liquid and solid phases of suspension into the dewatered sludge and sludge water. The sludge dry matter of dewatered sludge is in the range of 23-28%, and the sludge is in so-called digging state. Dewatered sludge is then loaded onto trucks and transported to a place of recovery.

Stabilised dewatered sludge represents a significant additional source of organic matter, nutrients and biologically active substances used for agricultural land. A fertilizer effect of the sludge is based on the favourable content of biologically important elements (N, P, K, Ca, Mg), sufficient organic matter and trace elements needed for good development of plants.

The final phase of sludge disposal is its use for agricultural purposes in the form of compost processed by an external contractor. Composting is recycling sludge in its own way because it allows return of substances to the natural food cycle.

Sewage gas

An optimally managed technological process of sludge stabilization ensures a formation of sewage gas kept at the volume of 400 – 450 litres per 1 kg of supplied organic solids. The sewage gas is essentially a mixture of methane (CH4) and carbon dioxide (CO2), which also contains small amounts of unwanted impurities such as nitrogen (N2), hydrogen sulphide (H2S), ammonia (NH3) and water vapour (H2O). The various components of the sewage gas are set out in table 1. In comparison with natural gas, the sewage gas has a lower content of methane (natural gas contains up to 98%), but significantly a higher content of CO2 (natural gas has only 0.07%). Of the ingredients, the most problematic is hydrogen sulphide because in a concentration above 0.1% has a corrosive impact on the engines and technological equipment. Ammonia is a source of odour.

Composition of the sewage gas:

Gaseous component of sewage gas Chemical formula Percentage [%]
Methane CH4 60 - 65
Carbon dioxide CO2 32 - 38
Water vapour H2O 0 - 10
Nitrogen N2 0 - 2
Oxygen O2 0 - 1
Hydrogen H2 0 - 1
Ammonia NH3 0 - 1
Hydrogen sulphide H2S 0 - 1

Combustible component of the sewage gas is methane, which is on one hand an excellent source of energy and, on the other hand, it is the second most important greenhouse gas. In the atmosphere, it captures heat 21 times more efficiently than CO2. Purposefully directed process of its capture, eventually directed process of its production and subsequent conversion to another gas and type of energy (e.g. electricity and heat) provides a benefit for society, while it contributes in a high degree to reducing the undesirable production of greenhouse gases, thereby protecting the environment.

Calorific value of sewage gas is depending on the methane content between 5.5 and 7 kWh/m3, about 6 kWh/m3 on average. The calorific value of natural gas is, due to significantly higher methane content, higher. Comparison of the most important parameters for the combustion gases is shown in table 2 below.

Combustion gases parameters

Unit Sewage gas Natural gas Propane Hydrogen
Calorific value kWh/m3 6 10 26 3
Density kg/m3 1,2 0,7 2,01 0,09
Density relative to air density - 0,9 0,54 1,51 0,07
Ignition temperature °C 700 650 470 585
Flammable range of gas concentration in air % 6-12 5-15 2-10 4-80

Production of electricity and heat

The produced sewage gas is piped from the overhead compartment of digestion tanks by pipelines to the gas holders (36). The role of gas holders is to ensure constant overpressure of the sewage gas throughout the gas system and compensate for irregularities in the production and consumption of sewage gas. The sewage gas is primarily used for the combined production of electricity and heat in cogeneration units (39) and partly for the production of heat in the boiler room (33). The majority of the heat thus produced is consumed in the processes of digestion; the rest is used for the heating of premises and preparation of hot water for WWTP. Produced electric energy is intended for the WWTP needs, which would otherwise have to be purchased from other suppliers. Obtained electricity and heat, on average, will cover more than 48% (for electricity) and more than 70% (for heat) of the total energy needs of the WWTP. The quantity of electricity produced is approximately 11,900 MWh per year, which is equivalent to the annual consumption of about 770 households.

The average quantity of produced sewage gas, electricity and heat is shown in table 3.

Balance sheet table of sludge and energy management of selected WWTP

Unit Vrakuňa Petržalka Devínska Nová Ves Senica Total
Number of inhabitants connected to WWTP EO 349 670 119 940 31 800 18 640 501 410
Unstabilised sludge Quantity m3/year 391 000 87 000 26 000 24 000 528 000
Sludge dry matter % 3,2 3,4 3,5 3,2
Quantity of processed dry matter t/year 12 512 2 958 910 768 17 148
Energies Sewage gas production m3/year 3 701 800 1 111 600 236 600 239 100 5 289 100
Electricity production MWh/year 8 840 2 340 300 430 11 910
Heat production MWh/year 10 260 3 190 990 770 15 210
Stabilized sludge Quantity m3/year 23 000 5 000 1 500 1 600 31 100
Sludge dry matter % 26 27 24 25
Quantity of produced dry matter t/year 5980 1 350 360 400 8 090