processes and techniques are used to clean up the biogas e.g. pressure swing ad/bsorption, membrane separation, cryogenic separation and water ad/bsorption. These processes will help to raise the quality of the gas to grid injection standards. Injection For gas to be injected into the UK gas distribution network, there are certain quality specifications that it has to meet. These are outlined in the Gas safety Management regulations (GS(M)R) which specifies the calorific value (CV) range for grid injected gas. However, since biogas naturally does not have as high a CV as required, it becomes necessary for the biogas to be injected with a higher CV gas such as propane to enable it meet the specified CV levels. However, the need for propane enrichment is more of a billing issue than a gas quality issue. The gas quality is then monitored downstream of the propane injection point and the signal fed back to the supervisory system to control the calorific value of the biomethane. The fiscal metering should be prior to the odorant addition. Prior to the injection of gas into the gas distribution network, the addition of odorant (80 per cent tertiarybutyl mercaptan, 20 per cent dimethyl sulphide) is required to impart the characteristic smell to the gas as required by GS(M)R. Routine monitoring carried out by rhinologists would confirm that acceptable odour intensity was being maintained. The odourization stage is hugely important from a gas safety point of view as too much smell would lead to more wrong gas