Household Waste as a Carbon and Energy Source

by D Seierstad, KanEnergi, Norway

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Introduction

From 1998, depositing organic waste at Norwegian waste disposal sites and landfills will become illegal. Consumer sorting of the waste will therefore be necessary. Apart from fodder production and composting, thermal hydrolysis is one way of making use of this potential organic waste resource.

 The hydrolysis product can be used as a carbon and energy source, directly replacing ethanol which is currently produced mainly from fossil fuels. The rejected material can be composted or digested for biogas production. A large-scale development project has been initiated to examine the possibilities of producing a hydrolysis product suitable as a carbon source in a microbial nitrogen treatment plant. Using low-temperature atmospheric hydrolysis and energy recovery systems creates an energy-efficient process. Using atmospheric pressure allows continuous hydrolysis and also reduces mechanical abrasion, the other main problem with hydrolysis.

Background

The municipality of Lillehammer was required by the environmental authorities to build a plant for nitrogen purification of its waste water in 1992. The plant started operating in the autumn of 1994, with a process making use of the reduction of nitrogenous compounds to nitrate with a further bacteriological conversion of NO2 and NO3 to free gaseous nitrogen (N2). The plant has a daily throughput of 15,000 m3 of waste water, releasing at least 75% of the nitrogen as gas. The aerobic process demands an external liquid carbon source. Until now, this source has been ethanol or methanol, which are readily available. The main problem with ethanol has been the cost of purchase, amounting to NOK 1.2 million annually (where NOK is the Norwegian krone). The Norwegian environmental technology developer Cambi A/S has been working on converting organic waste to marketable products, using hydrolysis and biological processes, since 1992. Cambi is co-operating with Lillehammer on this pilot project where a treatment procesfor household waste, as it is collected and sorted at Lillehammer, has been developed.

Aim

A test project was carried out in 1994 with the hydrolysis of 1,200 kg of wet organic refuse. It gave positive results, yielding around 50% as a readily available liquid carbon source, and the pilot project was initiated. The aim was to combine the nitrogen removal from the municipal waste water with the existing separation of the domestic refuse. Would it be possible to produce an alternative energy source to the ethanol, thereby saving the expense of buying ethanol, as well as reducing the area needed for composting? The project's success depended on supplying a carbon source which would provide nitrogen purification in compliance with the government's requirements. Surplus organic material in the reject was judged to be a suitable substrate for biogas production in a digester, and the project aim was expanded to produce a homogenous mixture of pure organic material appropriate for this as well.

Project outline

The construction of the hydrolysis plant and the testing of its product as a substitute for ethanol constitute the first of three linked projects. The second stage will investigate the possibility of energy production based on hydrolysis of Lillehammer's sewage sludge. The final part of the project is to build a full-scale prototype plant for joint treatment of wet organic waste and sludge, thus optimising the production of the carbon source and energy.

 The organic refuse collected by the inter-municipal waste management service includes organic domestic waste, disposable nappies, organically contaminated paper and organic waste from gardens. There is no further sorting of the refuse and the process design allows for a certain amount of erroneous material like metal, stones and plastic. The annual amount collected is 2,000 tonnes of domestic waste and 1,000 tonnes of garden waste, with collections in spring and autumn. This amounts to 40% of all the waste collected by the refuse disposal service. The nitrogen removal process consists of two identical parallel lines where the performance of the hydrolysis product will be compared to ethanol, under normal operating conditions.

The Plant

The household waste treatment plant itself is built into a standard 40-foot transport container, and will give valuable information on the performance of equipment suitable for the actual prototype plant. The NOK 2.5 million pilot plant is compact and easily transportable. This allows it to be used either as a small-scale production unit or as a test plant, at different locations, when the prototype plant is operational at Lillehammer.
 The organic refuse is collected from each household and stored on an asphalt-paved area. The hourly feeding of one tonne of waste to the container is done by a loader.
 The first step in the process is shredding which homogenises the refuse and increases the surface area to improve the hydrolysis. Hydrolysis takes place at a relatively low temperature, with continuous boiling and mild agitation at 90 - 100 C. Water is added during the process run of 30 to 120 minutes. The process time is regulated according to the refuse composition and the desired degree of hydrolysis. The organic material is broken down to volatile organic acids and taken out in the aqueous phase in a continuous dewatering press. The hydrolysis products are thus the liquid carbon substrate and the solid reject. The carbon source is stored in a tanker and is later injected into the basins at the purification plant. The reject is dealt with according to the level of organic content, either by direct deposit, re-separation or direct composting.

Economics

Lillehammer spends NOK 1.2 million annually on ethanol. It is intended that the ethanol will be completely replaced by the carbon substrate obtained from the refuse collected. The estimate for the cost of composting the organic waste is NOK 350/tonne, amounting to NOK 1.05 million a year. Currently, the compost is not a marketable product. There is therefore an additional benefit from turning the waste into an ethanol substitute as less money will be spent on composting. Not depositing organic waste will increase the life span of the waste disposal site by 10-15 years, postponing the NOK 200 million investment cost for a new landfill site.

Conclusions

The first phase of the project has shown that the technology is operational. Shredding and boiling homogenises the refuse, and the process separates the substrate and the reject well. A simple two-step separation showed that plastic can be skimmed off from the reject. Even with a high organic level in the reject, the volume of the refuse is reduced to 1/3 of the original. Prospects for the project are good, and although it is too early to conclude on the performance of the hydrolysis substrate, its production will solve a problem for Lillehammer and save the city some money too. Furthermore, combining the refuse and the sludge will allow surplus waste (not needed for carbon substrate production) to be used as an energy substrate together with the sludge itself. This opens up the potential to receive and make good use of waste from other regions creating centralised energy production.
 For more information contact the CADDET Norwegian National Team in Rud.

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