TITRE
TITRE


- The four investigated sludge disposal routes
- Methodology


The four investigated sludge disposal routes

We choose four different disposal routes on which to perform our case study. These four routes were

The first disposal route we investigate was incineration of the sludge.First, the sludge is dewatered before it is fed to an incineration plant at Shieldhall.We chose to investigate the energy balance for incineration because it is a technique which is used when sludge application to agriculture is not a possibility.
The only possibility for the energy recovery is a heat from the incineration process
All four routes start from the point where the sludge has been thickened to 6% dry solids, and ends at the point where the treated sludge has been transported to agricultural land or to landfill.

The second route involves both digestion of the sludge and incineration of the digested sludge.We chose to investigate this route because it combines a technique, where you can generate electricity and heat from biogas, with a technique where sludge application to land is not necessary.

Route 3 involves anaerobic digestion and then application of the sludge to agricultural land. The digestion takes place in two stages. First, the sludge is digested at a high temperature for 3 days. In this step the pathogens will be killed .Secondly the sludge is digested at a normal digestion temperature for another 12 days.
All the sludge is applied to land, but it is not realistic to apply all the sludge as liquid sludge. We chose to apply half of the sludge as liquid sludge. The other half was thermally dried to pellets before it was transported to farmers.

The last route of disposal is similar to the route proposed by West Of Scotland Water.The sludge is dewatered and is transferred to a thermal drying plant. The dry sludge is made into pellets before they are finally transported to Longannet coal fired power station and burned in mixture with coal
This route eliminates 2 problems. The first problem it eliminates, is the problem of getting planning permission for an incineration plant in Glasgow.
Secondly, you eliminate the problem of not having enough suitable land around Glasgow for land application of the sludge

Back to top


Methodology

Energy balances for the before mentioned sludge disposal routes was the basis of the evaluation of these four routes. The purpose of doing three energy balances was to see the effect transportation and the fertiliser value had on the disposal routes.

One set of energy balances was set up for the inputs and outputs of electrical & thermal energy and fuel used within the plant processes themselves. The value for the energy balances for the four routes, while only considering the plant processes, are shown in table number one.

Another set of energy balances was set up to take into account the fuel used for transportation of processed sludge to another part of the disposal route, like the transportation of dried sludge pellets from a thermal drying plant to Longannet Coal fired power station or to farm land. For this energy balance the results are shown in the second table.

The final energy balance is an energy balance including transportation and fertiliser value of the sludge applied to land, and the results are shown in the larger third table.

The energy balances for the plant-processing only This energy balance includes the energy inputs in the form of thermal & electrical energy used in the plants, where the efficiency of transferring heat is taken into account. In the case of thermal drying, the energy inputs also include the fuel used in the drying plant. The energy inputs does not include the power demand to pump the sludge between elements of the plants or the use of heat and power needed in office buildings and other site buildings. The energy outputs consist of the electricity and heat which can be recovered from burning biogas in an engine or the heat which can be recovered from elements of the thermal drying or incineration plants. Only in the case of Longannet power station have we not considered the potential heat to be recovered from the incineration of the sludge pellets, since this plant is an existing plant where there is to our knowledge no plans of implementing heat recovery.

The energy balances including transportation In this part of the case study the additional energy requirements to transport processed sludge to a second site for further treatment or to the final site of disposal are included. This means for route 1 & 2 that they include the fuel consumption of transporting ashes to the final disposal at the landfill site. For route 3 the fuel used for bringing liquid sludge or sludge pellets to the farmers is included. And for route 4, the fuel used for transporting the dried pellets from Shieldhall in Glasgow to Longannet Power Station will be included.

The energy balance including transportation and fertiliser value. When the sludge is applied to soil in the anaerobic digestion with land application of the digested sludge route, it can be seen as a way of recycling a valuable resource in the form of nutrients. In order to estimate this effect, we assumed that the energy value of the digested sludge can be represented by the energy consumption of producing the fertiliser that the sludge is substituting. Therefore this final energy balance takes into account the energy saved on the production of ammonium nitrate, which is a cheap nitrogen fertiliser.

We allowed for capital cost by estimating the cost of building the plants alone. Running cost were not considered at all.

The way environmental and public concerns were taken into consideration was to shortly name the main concerns for each technology.

The reason for considering these other issues is that only by getting an overview of the full picture, is it possible to appreciate the complexity of the problem, we wanted to solve.