Thermal Hydrolysis of Waste Water Sludge

by the CADDET Norwegian National Team

INTRODUCTION

At Hamar in Norway 13,000 tonnes sludge/year is continuously converted at the municipal sewage treatment plant (HIAS). The process gives a 93% reduction in sludge volumes and 15,000 kWh/day net energy production in the form of steam at 10 bar pressure.

The HIAS plant uses anaerobic digestion for biogas production. Thermal hydrolysis increases the biogas production by up to 50% compared with conventional digesters. The hydrolysis makes the sludge readily available for digestion dissolving and decomposing the solids; it also facilitates a higher degree of separation of the solid and liquid phases.

The plant also recovers half of the precipitation chemicals used in the waste water treatment plant by centrifugation leaving a residue with a total solid content as high as 40%, which reduces the energy required for incineration or drying. The final output after drying amounts to 7% of the original volume and is used as cover material for recultivating the local landfill site.

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The process reactor for continuous digestion of volatile solids

PROJECT BACKGROUND

Dewatered sewage sludge is often regarded as a problem waste; in addition to expensive handling of large volumes, sludge may contribute strongly to water pollution and the release of greenhouse gases, organic pollutants and heavy metals.

In 1991 HIAS, the municipal waste water treatment company in Hedmark County in Norway, initiated a project with Cambi AS to develop a sludge treatment plant. A pilot treatment plant, based on thermal hydrolysis and capable of treating the sludge from a community of 10,000 people, was built and showed the effects of the different process parameters. The main objectives for the project were:

  • to minimise residual sludge volumes;
  • to determine the properties of the treated sludge, with respect to the potential for energy production;
  • to determine the possibilities for upgrading the treated sludge to marketable products.

The pilot plant resulted in patented technology, able to convert the dewatered sludge to a range of useful products: biogas, precipitation chemicals, carbon sources and cover material. Against this background, the construction of a full-scale treatment plant was started in 1994. The full-scale plant was put into operation in November 1995 and processed 2,660 tonnes total solids (TS) from 13,000 tonnes sludge in its first year of operation. The final amount of residue to be disposed of was 2,000 tonnes before drying (with drying this is reduced to 1,000 tonnes). The amount of total solids in the residue is 800 tonnes. Raw sludge is dewatered in a centrifuge and fed into the hydrolysis process.

THERMAL HYDROLYSIS

The thermal hydrolysis part of the process consists of two pulping tanks, one reactor tank and one flash tank. The sludge is fed into the pulping tanks where it is homogenised, preheated and cut to a particle size of less than 3 mm in a macerator pump. It is then passed to the reactor tank for thermal hydrolysis by treatment at high pressure (around 10 bar) and temperature (180 C) for 30 minutes. This process leads to the conversion of complex organic material such as proteins, lipids and cellulose fibres, into simpler chemical substances readily available for digestion. The pressure and temperature are reduced in the flash tank.

To minimise the total energy consumption, a heat exchanger transfers heat from the flash tank back to the pulping tank to assist the preheating. Earlier thermal hydrolysis processes gave problems with wear on the valves; this problem has been minimised by reducing the temperature and the pressure stepwise.

DIGESTION

The hydrolysis process dissolves 27% of the (chemical oxygen demand) COD in the sludge. The cooled sludge is fed continously into the digester where anaerobic fermentation converts up to 62% of the volatile solids to biogas. Fermentation of non-hydrolysed sludge converts in comparison 30-50% (of the total energy content). The 1,400 m3 digester is producing around 3,000 m3 biogas per day, with a methane content of 65%.

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Schematic of the HIAS System.

ENERGY BALANCE

The biogas is used for steam production used for the hydrolysis itself (26%) and in the local HVAC system (74%). The operational data for January 1997 depicts the plant running under normal conditions and gives a picture of the energy balance of the process.

Gas production in January was on average 2,563 Nm3/day with a 65% methane concentration. This produces 16,600 kWh/day gross and 15,000 kWh/day as net energy in the form of steam at 10 bar pressure. The process energy consumption depends on whether the sludge is used directly for farm land recultivation or whether the extraction process and dryer is in operation. In January the former was the case giving the energy balance shown in the green box.

With the dryer operating, the overall energy balance is not altered much as the heat is recovered and used in the hydrolysis process.

Energy production and consumption in January 1997

Live steam for the hydrolysis process (average): 8,850 kWh/day
Recovered energy at flash tank heat exchanger (average): 6,125 kWh/day
Net energy consumption for hydrolysis: 2,725 kWh/day
Energy to HIAS for HVAC system: 15,000 2,725 = 12.275 kWh/day
74% of the gross biogas production is utilised for HVAC

END PROCESSING

Recycling of the precipitation chemicals from the waste water treatment plant is important for the economy of the plant. Final centrifugation and the drying process leaves a stable end product with around 90% TS and a volume of 7% of the input.

Several marketable products emerge from the process. The liquid phase of the hydrolysed and dewatered sludge can replace methanol as carbon source in a biological nitrogen removal process. The dried residual sludge can be used as cover material on landfill sites or other recultivation purposes. The nitrogen in the sludge can be converted to fertiliser. The plant has an excess capacity of about 1,000 tonnes TS per year. Sludge from other municipalities can be treated and the plant is designed to accommodate a gas motor for cogeneration. This will produce electricity for the grid and also cover the steam requirement of the hydrolysis.

PERFORMANCE

After an initial start-up period, the plant has consistently treated the full amount of sludge produced, operating around its design capacity of 57 tonnes per day.

The volume and the TS weight is reduced throughout the process. The TS content in the raw sludge is 20%, equalling 2,660 tonnes/year. The TS weight is reduced by 70% through the plant and the overall handling volume is reduced 93%. More than 50% of the precipitation agents are recovered and reused.

For more information contact the CADDET Norwegian National Team at Rud .

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