The Freeze Dry Unit allows for savings in many ways. For example, the Freeze Dry Unit uses less energy than other drying methods today, which lowers energy cost. The process does not require any polymers which completely reduces costs associated with polymers. The final product has a high dry solids content that lowers costs from transports.
A wastewater treatment plant produces 20,000 cubic metres of sludge with 2 % DS and uses 12 cubic metres of polymers a year. Sludge dewatering is operational for 5,200 hours each year. The plant uses a screw press to dewater the sludge. Transports of polymers are from France and the cost of polymers are € 24 221 a year. The distance to the disposal location is 80 kilometres. All dewatering and drying methods have a lifespan of 15 years and the discount rate used is four percent. The price of energy is € 0,07 per kWh.
The screw press has an annual energy cost of € 678 as it only uses 2 kWh (with addition of pumps for polymers) and dewaters the sludge to a DS of 25 percent. The transportation cost is € 130 424 a year when the sludge is transported from the treatment plant to the disposal location. In this case, the service cost is estimated to be € 9 316. No investments or installation costs are considered as the treatment plant already has the screw press in place. The total life cycle cost is € 1 830 532, which equals to € 4,38 per kg DS.
If the treatment plant wants to achieve a higher DS, up to 95%, they can buy a belt dryer. The combination of a screw press for dewatering and a belt dryer for drying has an annual energy cost of € 96 613. The increase in energy is due to the extra energy that is needed to evaporate the water in the sludge. This belt dryer uses 915 kWh per tonne of evaporated water including the electricity needed for support functions. The investment of a belt dryer is € 1 257 662. Transports between treatment plant and the location for disposal are reduced to only € 26 084 due to the decrease in sludge volume a dryer sludge product results in. In this case, the installation cost is estimated to be € 5 589 and service costs are estimated to be € 27 946 a year, excluded of the service costs of the screw press. The total life cycle cost is € 3 301 343, which equals to € 7,92 per kg DS.
If the same treatment plant instead chose the Freeze Dry Unit, the annual energy cost would only be € 67 820 and directly dewaters and dries sludge between2 and 95 percent DS. No previous dewatering method or polymer addition is needed, so there is no polymer cost or transportation of polymers. The transportation cost between treatment plant and the location of disposal is only € 26 084, due to the decreased sludge volume. The installation cost is only estimated to be € 931* and the service costs are estimated to be € 9 316* annually. The total life cycle cost is € 2 275 936, which equals € 5,5 per kg DS.
*These costs could change