Forebyggelse af lugt og andre barrierer for biogasanlæg

Summary

There is no reason why biogas plants should smell so bad.

Biogas plants will never be completely odourless. Systematic planning, however, may prevent major nuisances from a specific biogas plant. The important thing is to close in all odour sources as well as to accumulate and purify extraction air effectively.

Background and aim

Finding the site for a new biogas plant is often met with strong opposition, among other reasons the because of fear of smell. The Environmental Protection Agency (Danish EPA) has published a report on prevention of smell and other barriers for biogas plants. Building on the experiences from existing plants it aims to describe how future plants may be constructed to cause the least possible nuisance for the neighbours.

Neighbours of biogas plants often see smell as the dominating nuisance but when new plants are being planned, elements which are often mentioned are traffic and decline in real-estate value.

There is a distinction between normal and abnormal nuisances from a biogas plant. Abnormal nuisances occur a few times a year in connection with maintenance or mishaps in a plant. Normal nuisances are experienced more often in the course of normal running of the plant. Only normal running is regulated by permit limits.

The neighbours experience the odour nuisances as a combination of intensity of smell (weak, clear, strong), of the type of smell (slurry, burnt, rotten, sweet), if it is pleasant or unpleasant and how frequently and for how long the smell appears. It is not possible to calculate the odour nuisances which the neighbours experience. Instead, the authorities must decide permit limits on the basis of a simplified model in which they only consider the total amount of odour concentration during normal running. The odour concentration indicates how many times an air sample may be diluted to render it odourless. It is described in odour units (OU) per cubic meter.

The Danish EPA recommends that the odour concentration in residential areas does not exceed 5-10 OU/m³ outside the boundary line of the enterprise. In actual open spaces this concentration may be increased by a factor 2 -3 in certain areas. Different contributions from the same enterprise must be included. The Danish EPA has set guidelines for a practice that the contribution of any enterprise to smell in the surroundings must not exceed 10 OU/m³ in areas of commerce, industry and agriculture, and 5 LE/m³ in purely residential areas. The OML model is employed for this.

The Investigation

Problems and solutions concerning odour etc. have been investigated in 11 biogas plants in Denmark. A real estate agent has been contacted near every plant to learn whether property prices have been affected by the proximity of a biogas plant. Furthermore the municipal environment supervision body was contacted to learn of their experience with the plant

In 10 out of the 11 plants visited, the ventilation air from reception tanks and possible loading hall is cleansed in a bio filter. The filter standard varies from simple, open bark filters to advanced bio filters with monitoring of moisture and pH as well as high yields.

Main conclusions

Most existing plants have or have had problems concerning smell. On the other hand house prices do not decrease when biogas plants are being constructed nearby, and traffic does not give serious problems.

The worst odour problems in biogas plants are caused by leaks and outlet of insufficiently treated process air or unintentional gas emissions. In a new biogas plant the sources of smell, requiring particular attention are, unloading of solid strong-smelling biomass and emissions of air from reception tank and reception hall.

Biogas plants will never be completely odourless but systematic planning may prevent important nuisances from a specific biogas plant.

Planning comprises reviewing the site, use of strong smelling biomass, possibilities for transport, unloading method, ventilation from reception tanks and loading areas, tank materials, cleansing and dissemination of extraction air as well as procedures for running and maintenance.

Project results

Recommendations for a future biogas plant

The experience from the existing plants can be collected in a series of recommendations for future plants.

Odour from containers should be kept confined or collected and cleansed.

Reception tanks should be airtight and should be constructed to allow a partial vacuum to be maintained, even during input of biomass.

Membranes over reactor and gas tanks, where a pressure above the atmosphere exists, should be odour tight. (Odour may permeate through a plastic membrane which is considered gas tight.) Long retention time and well functioning processes ensure that most odour substances have degenerated before the degassed slurry leaves the plant.

Slurry and liquid biomasses as well as degassed biomass may be unloaded in a closed system that includes cleansing of extraction air.

Solid, strong smelling biomasses should be unloaded in a closed hall where ventilation ensures partial vacuum.

Ventilation plants should ensure constant partial vacuum in the unloading hall, reception tank and other places where odour occurs in order to avoid diffuse odour emissions. For example, ventilation air may be cleansed in a bio filter. The bio filter should be dimensioned to the given max. amount of air and there should be clear instructions concerning control, operation and maintenance.

Expulsion from both filter and the gas engine should be dimensioned to observe the given conditions concerning odour.

Operation and maintenance of the total plant should be planned carefully. As far as possible, routine maintenance which may cause odour should be carried out during the cold season with an appropriate wind direction.

Furthermore, areas and lorries/cars should be kept clean.

It would be sensible to keep a good distance to neighbours. Although a new plant is dimensioned to not cause nuisances to the neighbours, unintended nuisances are bound to occur. A fair estimate for distance would be 500 m to housing and other residential areas. The distance to single dwellings in the open countryside should be estimated case by case.

Establishing pipe transport of liquid biomass may be considered if the transport to the plant affects residential areas considerably. So far there is no experience in the biogas plants visited concerning investigation of odour nuisances in connection with pipe transport.

Small expenses for prevention of odour

Initiatives to prevent odour may be carried out at different levels and it is difficult to separate costs of odour prevention from costs of the rest of the plant. A new joint biogas plant for 180,000 m³ biomass per year may typically cost app. DKK 70 mill. in 2006. Only a very small part of the construction costs need to be spent on preventing odour nuisances. Similarly, the operation costs for a possible unloading pit or similar initiatives, extraction and cleansing would only make up a small part of the collected operation expenses.

Systematic reporting

When giving an environmental endorsement the authorities are recommended to request systematic reporting of nuisances from odour and transport as well as procedures for prevention of the nuisances. The report should contain a detailed review of all predictable odour sources with analyses of all operation situations and how these are handled in order to avoid odour emissions. The same report would also be a help to target later rectification in case of unforeseen problems occurring in the operational phase.

Perspectives

So far odour nuisances have been rectified after the construction of the biogas plant. The significance and the strength of each odour source have not yet been sufficiently examined. Therefore all calculations in this report have been carried out from a worst case scenario. Only when the solutions have been tested in practice will we know if they are adequate.

There is need to continue collecting experience concerning nuisances caused by biogas plants.

There is a particular need for measurements of each odour source, different filters (before and after cleansing), different gas engines and an assessment of the affect of possible diffuse odour sources on the surroundings.

Besides this, there is a need for development and collection of experiences concerning solutions for unloading solid garbage, ventilation systems and principles for cleansing.

As experience with odour grows and odour measurements are carried out on discharges and emissions, a better basis will be created for optimizing the initiatives to suit the actual conditions concerning odour and adjustment of ventilation, cleansing and emission.

Future biogas plants will generally be bigger than in the past. Concerning odour control in future plants there is a need for a generally better standard. The solution does not only concern concrete initiatives.

There is an explicit need to increase in focus and systematic in connection with project planning and establishment of new plants in order to make odour control efficient from the very start of the plants. Measures to be taken to control odour must also be well described and include instructions in order to ease the follow-up and number of faults in the operation phase.

Under all circumstances the cost of odour reduction is minimal compared to the fact that it may be crucial for approval of a specific biogas plant to be situated in the area and to avoid nuisances from the plant in the operation phase.

 



Version 1.0 Januar 2007, © Miljøstyrelsen.