Digestion of sludge - Degremont

Degrémont has a tradition of sharing its employees’ passion for water treatment with the public. To supplement the Water Treatment Handbooks, Degrémont has issued the «Handbook Factsheets» to promote a better understanding of the different techniques available and discovery of the new products and major technological changes.

Degrémont Water Treatment Handbook Factsheets

Biosolids

DIGESTION / CODIGESTION OF SLUDGE: RECOVERING THE ENERGY POTENTIAL OF SLUDGE GENERATED BY THE TREATMENT OF WASTEWATER

Sludge treatment

Non dewatered sludge

Additional local opportunity for communities looking for sustainable energy solutions and a potential source of income, the digestion of sludge from wastewater treatment, once considered as a sludge management tool, is now seen as an energy transition tool: more than 40% of the potential energy from sludge can be recovered through the biogas produced. The management of sludge from wastewater treatment plants is strongly impacted by the level of development and the planet’s rates of urbanisation: the more wastewater we treat, the more sludge we produce. It is without doubt the wastewater treatment affiliated area that will require the most improvement over the coming years: treating sludge whilst reducing the quantity produced, obtaining better-quality and healthy sludge, taking into consideration the economic reality of the increase in treatment costs in proportion to the increase in volumes and the required improvement in quality. The constraints represented by the increased volumes of sludge to be treated are offset by the enormous energy potential of such sludge through the produced biogas. It can be recovered to provide energy to a wastewater treatment plant. The digestion of sludge from wastewater treatment therefore contributes to achieving the objectives set by the applicable regulations. Europe, for example, has committed to drastically reducing its greenhouse gas emissions with the target of producing 20% of energy from renewable sources by 2020.

DIGESTION

In the same way as lime treatment, digestion forms part of the sludge stabilisation process. It uses the fermenting capacity of organic matter (OM), i.e. the most easily biodegradable components, to eliminate them in large quantities. At the same time, the digestion performs a more or less intensive sanitisation function (reduction of pathogenic organisms) There are two main types of digestion: - Aerobic digestion; - Anaerobic digestion.

AEROBIC DIGESTION • Mesophilic aerobic digestion: this occurs at temperatures of 35–40° C in the presence of oxygen (O2) and requires specific submerged aerators to be installed in the digester, often combining the introduction of fine air bubbles and mechanical stirring. The minera-

Digestion

lisation rate obtained depends mainly on the quantity of O2 transferred, the retention time, the temperature and the age of the sludge introduced. In the most common climatic conditions, the reduction of the amount of OM in the produced sludge is about 20%. It is possible to achieve 25–45% through longer retention times and especially through very high quantities of transferred O2. These performance levels are more easily achieved at facilities that enable thermophilic conditions to be maintained. • Thermophilic aerobic digestion: this technique uses the exothermic nature of oxidation reactions to increase the temperature (45 to 60° C) in the digester. This temperature rise increases the reaction rate and may, if it is sufficient, allow for a degree of sludge sanitisation. To obtain an adequate temperature in the digester, arrangements must be made to limit heat loss. Foam production must also be controlled, along with the occasional emission of odours. In practice, maintaining aerobiosis conditions often requires extra heating.

ANAEROBIC DIGESTION Anaerobic digestion was used for the first time in Exeter (United Kingdom) in 1885, almost 130 years ago. It involves the transformation of sludge through bacterial action in the absence of O2, a process known as methane fermentation. • Principle The fresh sludge injected into the digester is mixed mechanically or through the insufflation of a digestion gas to increase the contact between microorganisms and the OM to be degraded. There must be a regular supply of fresh sludge. The influence of the temperature is vital in the smooth functioning of anaerobic digestion in terms of start-up speed, stability of the fermentation, and so on. Mesophilic fermentation (35–40°  C) is generally the option implemented. Thermophilic fermentation (45–60°  C) enables the digester volumes to be reduced by up to a factor of 2 and allows for more effective removal of pathogenic germs. The retention time and initial OM concentration are key factors for the synthesis of bacterial flora. In addition to the temperature, the quality of the digestion process is also dependent on the retention time, the stirring intensity, the regularity of supply and the OM (type, structure and content).

Two-phase anaerobic digestion is an excellent compromise. It consists of rapid OM hydrolysis in an initial thermophilic reactor with a low retention time, followed by an optimisation of the methane fermentation process in a second mesophilic reactor.

CODIGESTION On an industrial scale, codigestion implements in the digester both sludge from wastewater treatment (primary substrate) and organic biowaste with high methanogenic power (unsold supermarket goods, fermentable household waste, fats, liquid manure, etc.). These secondary inputs ensure a constant supply of OM and improve biogas production.

 Biogas production Anaerobic digestion produces biogas formed mostly of carbon dioxide (CO2 – 35 to 50%) and methane (CH4 – 50 to 65%). The biogas generated has the advantage of being a storable source of energy which can be recovered in various ways (cf. § Biogas recovering solutions) The biogas produced by a wastewater treatment plant may be optimised in different ways: - Use of mixed sludge = primary sludge is more fermentable than biological sludge. The yield of biogas produced through the digestion of mixed sludge, a mix of the previous two, is better compared to a digestion of biological sludge alone; - Degradation by sludge hydrolysis before digestion = hydrolysis increases the release of OM and thus the production of biogas; - Codigestion (cf. § Codigestion).

 Advantages of anaerobic digestion

In addition to the improvement of biogas production, codigestion carries the advantage of accelerating the profitability of the digester investment and then represents a source of income for the community, as industry will pay for the treatment of their liquid waste.

BIOGAS RECOVERING SOLUTIONS  Heat recovery With a boiler, the recovery of heat from the combustion of biogas can be used to: - heat digesters and premises; - supply dryers with thermal energy.

 Cogeneration

• Technical - Sludge stabilisation: the digested sludge gives off a mild odour, it is homogeneous, its outdoor handling does not cause nuisance and there is no return to fermentation, even after a lengthy storage time; - Sludge volume reduction: fermentation leads to the removal of large quantities of OM. In urban wastewater treatment plants, the reduction of OM content in mixed sludge may be as much as 40 to 50%, which corresponds to around 1/3 of dry solids; - Reduction of downstream dewatering facilities (1 to 3 points improvement in dry solids content for dewatering compared to fresh sludge). • Health - 90% of salmonella and the majority of pathogenic germs are destroyed (viruse and helminth egg removals are slightly less effective). • Environmental - Recovery of organic matter as green energy: the biogas produced contributes to the energy self-sufficiency of wastewater treatment plants; - Reduction of environmental impacts linked to transportation and future of this sludge (resulting from the volumes reduction); - Improvement in the environmental and carbon footprints of facilities; - Reduced fossil fuel consumption. • Economic - Optimisation of the sludge treatment equipment design due to the reduced volume of sludge (dewatering, reagents, drying, etc.) = limiting the investment of the sludge and odours treatment lines; - Operating cost reduction for sludge treatment; - Resale of renewable energy at a preferential tariff.

From biogas motors or turbines, cogeneration allows for the simultaneous production of: - electricity that can be used on-site or reinjected into the network; - heat that can be recovered on-site for heating premises and digesters, and supplying dryers.

Cogeneration therefore improves the wastewater treatment plant’s energy breakdown and its carbon footprint (by preserving fossil fuels), and can be a financial resource for the community through the resale of electricity.

 Injection into the natural gas network Biogas contains carbon dioxide (CO2), hydrogen sulphide (H2S) and other pollutants. Once these compounds removed, biogas can be converted into biomethane, a gas composed of over 97% methane (CH4), with a composition similar to that of natural gas. This biomethane may therefore be injected into natural gas distribution or transport networks.

 Biofuel production The biomethane recovery as fuel requires an advanced purification similar to that needed for injection into networks. Depending on the type of biofuel produced, there are two possible uses: - fuelling urban transport vehicles; - fuelling long-distance transport vehicles (boats, trucks).

Degrémont Water Treatment Handbook Factsheets  Digelis™ fast

When used as a fuel, biomethane reduces noise pollution by 50% in comparison to diesel engines and reduces vehicle vibrations. In terms of public health, these biofuels do not emit fine particulates and reduce nitrogen oxide (NOx) emissions by 80%.

DEGRÉMONT’S DIGESTION OFFER An expert in water treatment and a driving force in the circular economy and in recovery, Degrémont, a part of the SUEZ ENVIRONNEMENT group, is acting to develop innovative and quality wastewater treatment solutions by focusing on its objective of improving an offer which considers wastewater treatment sludge from its production right through to its recovery. First builder of digesters in France, Degrémont has forty years of expertise in digestion.

THE RANGE

INNOVATIONS  Digel Digelis™ Digelis is™ ™ Smart

This optimised sludge digestion workshop implements a thermophilic digestion process combined with an energy recovery equipment at the digester outlet: this layout accelerates the sludge digestion cycle while keeping the same energy consumption as mesophilic digestion. Performance - Doubling of digestion reaction kinetics; - 40% reduction in the total ground area footprint of the structure due to the accelerated digestion kinetics; - Decrease of the sludge retention time in the structure by up to 40% compared to a conventional mesophilic process.

 Digelis™ Duo In conventional anaerobic digestion, acidogenesis and methanogenesis phases (the acid forming and methane forming phases) occur in the same digester. There is a loss of process efficiency because the acid forming bacteria do develop much faster than the methane forming bacteria. With Digelis™ Duo, these two phases occur in separate digesters in series. The first phase carries out at thermophilic conditions (55 °C) with a retention time of two to three days and the second phase at mesophilic conditions (37 °C) with a retention time around ten days. Thermophilic temperatures maximize pathogen destruction. Performance - Biogas production from the mesophilic phase will be significantly higher than from conventional anaerobic digestion; - The total retention time is reduced compared to conventional anaerobic digestion. This means that total digester volumes can be reduced and/or sludge load rates can be increased; - Nocardia bacteria are the cause of most digester foaming int the case of mixed primary sludge. These mesophilic organisms are killed during the first thermophilic phase; - The two phases use the Degremont’s Cannon Mixer® system for the mixing, which ensures an active digestion volume over 90% of the total digester volume.

 Digelis™ Turbo

Digestion is boosted by thermal (165° C for 20–30 minutes) hydrolysis* of the biological or mixed sludge, upstream of the mesophilic anaerobic digestion process. * technology developed by Cambi

This mesophilic digester for urban sludge is designed for plants serving 15,000 to 100,000 population equivalents. Digelis™ Smart combines digestion and biogas storage in a single chamber. Performance - Ground area footprint reduction.

Performance - Dry solids content improved by 4 to 8 points in comparison to a traditional digestion process; - Production of recoverable biogas increased by 50% with biological sludge; - Autothermicity of the digested biological sludge before incineration; - Possibility to double the capacity of the digesters at an existing plant; - Sludge reduction costs are reduced by 50% for a new plant.

Degrémont Water Treatment Handbook Factsheets  BioGNVal

 LA FARFANA (Santiago, Chile) – 4,000,000 PE

In order to produce liquefied biomethane fuel (Bio-LNG) from biogas at wastewater treatment plants, in partnership with the young innovative company EReIE, Degrémont is developing a biogas purification and liquefaction technology using cryogenics. The Bio-LNG produced will be of a sufficient quaPhoto credit: SIAAP lity to allow for the same industrial and fuel uses as those of natural gas or liquefied natural gas (LNG). Industrial testing, on a pilot scale, will be performed at SIAAP’s Seine amont wastewater treatment plant in Valenton, France. This project is funded by ADEM, the French environment and energy management agency.

A FEW REFERENCES...

Digestion by Digelis™ / Heat recovery and injection of biogas into the network

- Digestion of 200 tonnes of dry matter per day, - Production of 80,000 Nm3 of biogas per day, - Boiler use of 20,000 Nm3 per day, - The remainder part is purified before being injected into the nearby natural gas network.

 CHOLET (FRANCE) – 116,000 PE Digestion by Digelis™ / Codigestion

Digestion of sludge from wastewater treatment and: - Abattoir sludge; - Fats; - Waste from the agri-food sector.

 AS SAMRA (Amman, Jordan) – 5,100,000 PE Digestion by Digelis™ / Cogeneration

 MAPOCHO (Santiago, Chile) – 3,500,000 PE

- Heat and electricity production (10,000 kWe after upgrade).

Digestion boosted by Digelis™ Turbo / Cogeneration

 MEISTRATZHEIM (Obernai, France) – 204,000 PE Digestion by Digelis™ Smart / Cogeneration

- Digestion of sludge from wastewater treatment and sauerkraut juice; - Heat production for sludge drying; - Electricity production (190 kWe).

 LA FEYSSINE (Greater Lyon, France) – 300,000 PE Digestion by Digelis™ / Heat recovery

- 4,187 m3 of biogas produced per day; - Recovery of the associated biogas heat burned in the boiler for heating up the digesters and for supplying the thermal sludge dryer; - 77% of the dryer’s needs are covered by biogas recovery; - Natural gas saving of around 950,000 Nm3/year.

 Chattanooga (Tenessee, USA) – 1,600,000 PE Digestion by Digelis™ Duo / Heat recovery

- 2 thermophilic digesters + 4 mesophilic digesters; - Recovery of the associated biogas heat burned into boiler for the sludge treatment needs.

 STRASBOURG (FRANCE) – 1,000,000 PE

Digestion by Digelis™ / Injection of biogas into the network

- Biovalsan = the first project in France injecting the biomethane into the network; currently the biogas acts to supply the incinerator (7,200 Nm3 per day).

 Marseille (FRANCE) – 1,800,000 EH

Digestion by Digelis™ Fast / Cogeneration

 FOLSCHVILLER (FRANCE) – 20,000 PE

- 27,000 Nm3 of biogas produced per day; - Electricity production (19,000 kWe).

Digestion by Digelis™ / Cogeneration

- Plant upgrading with an additional primary settling and a digester; - Heat and electricity production (28 kWe).

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Degrémont Handbook Factsheets n°9 - June 2014 - Photo credits : Degrémont

- Using the four existing digesters and building of an additional one (15,000 m3); - 6% increase in electricity production; - 27% decrease in sludge production; - Final product recovered in agriculture; - The biogas produced is used in cogeneration to provide the heat required by the Digelis™ Turbo and supply electricity (8,100 kWe).