Before discussing the conclusions we reached through considering our energy balance results and other peripheral issues, an explanation of why we choose to show the given values in the result tables will be given.

We have shown three net energy results for each route in the bottom table. One for electrical, another for thermal and the last for fuel all in GW-h. We did this with the aim of observing what type of net energy was being used or produced for each route. Thermal energy recovery is only of value where there is a local outlet for the excess heat such as in the process itself or a nearby industrial or urban area. Any route that only has heat recovery and does not utilize that energy becomes a disposal option rather than an energy recovery option. Electrical energy use is not limited to the immediate area, it is therefore a more profitable resource and the preferred type of energy to be produced from a process.

The bottom figure shown in each table is the Coefficient of performance or COP value. It is the sum of the energy outputs divided by the sum of the energy inputs. A figure greater than one is the most desirable result as this means the disposal route is energy productive. COP is used because it is the most effective method in comparing various routes from an energy balance point of view.

Discussion

The COP values, shown the first table, for routes one and two show that they are energy intensive processes. The digestion followed by land application routes energy balance breaks even and the Refuse derived fuel (RDF) incineration route is energy productive as indicated by a COP value greater than one.

If you now look at the second table that includes the energy used for transportation as well as for the plant processing we observe that the COP values change but only nominally, therefore transportation is of little importance when considering the energy balance for different disposal options. This came as a surprise to us as we thought that transportation would have a significant effect on the energy balance.

The last table includes the fertiliser value for route three. The COP value for route three has change from that shown in table two, this would indicate that the fertiliser value does have a relatively important effect on the overall energy balance for land disposal, however this value is an estimation of a potential fertiliser saving for the farmer and not an energy input which necessarily benefits the local water authority. It therefore cannot be considered as significant as the other energy outputs.

If you again look at the bottom table you will observe that for route one the inputs are three to four times greater than the outputs. This is because so much electrical energy is needed in the process yet there is no electricity produced from the process. The only energy recovery we have is thermal and is from the flue gases, most of this however is used in the process itself, and as mentioned earlier excess thermal energy is of limited value.

Route two's coefficient value is still relatively low, again because of the electricity used in the incineration process, which far outweighs the electricity generated from the combustion of the digestion biogas.

Clearly both routes are inadvisable unless their is no other disposal option for the sludge, such as land application or incineration of refuse derived fuel (RDF). These options may not be possible where there is not enough agricultural land suitable for land application or because of the financial constraints of incineration of RDF. These constraints are potential problems in securing contracts with local Coal fired power stations and the comparatively high capital cost of this disposal option.

It can be seen from the table that route three's inputs almost equal the outputs, however the COP value would be greater if it were not for the fact that we choose to thermally dry 50% of the digested sludge to produce a more marketable product. The fuel consumption is only for the 50% thermally dried sludge and not for any other part of the process. In addition the pasteurization step in digestion is energy intensive.

Digestion followed by land application is considered the best practicable environmental option for sewage sludge in most cases because its the only practicable recycling disposal route for sewage sludge and the biogas from the process can be used for electricity production and heat recovery, however the availability of suitable land is too small to ensure a sustainable outlet in the Shieldhall area.

There is a 13% greater output than input for route four, making it the most energy productive. It is therefore our groups recommended disposal route and is similar to the most probable disposal route that the West of Scotland Water will implement after the end of this year.

• To summarize, it can be seen from the final table why the other routes are less effective than route four.
  
• Routes one and two are energy intensive and it came as a surprise to us after we had investigated dedicated incineration how energy intensive it actually is to incinerate sludge rather than solid waste, in addition because of public prejudice there are difficulties in obtaining planning permission.
  
• Route three is energy productive and is the only route that recycles the sludge, nevertheless there is a problem with land availability and we believe there could be public concern over pathogens.

• The only problem that we see with route four is the high capital costs, however this high capital cost figure is based on an existing plants costs and doesn't take into account the savings in capital costs that would be made with the increase in plant size, whereas the other routes capital costs are estimates based on a generalized formula and cannot be considered as accurate as that of route four.
  Consequently we don't think the high capital cost figure calculated for route four is sufficient grounds to reject this route as our recommendation for the disposal of sewage sludge from the Shieldhall sewage treatment works.