Background It is estimated conservatively that each EU citizen produces 12-18 g of sludge - dry weight - from biological waste every day. Treatment systems typically involve processes of: 1. concentration by sedimentation and flotation; 2. stabilisation by anaerobic, aerobic or chemical treatment; and 3. natural drying, or a filter press or vacuum drying with centrifugation. The most common form of re-use for sludge waste is in agriculture, accounting for around a third of sewage sludge in Western Europe and a slightly higher figure of 38% in Eastern European countries. However, the economic costs of effective sludge treatment are relatively high. Unfortunately, most sludge waste is simply disposed of in landfills or dried in an ineffective way and incinerated, resulting in low efficiency because of residual high water content (30%). Small treatment plants - located in areas populated by less than 100 000 inhabitants – particularly struggle to make treatment processes cost effective. In many cases the waste has to be transported at relatively high cost to a suitable plant. Treatment of sludge waste in contexts such as an offshore oil rig present even more specific challenges because of inefficiencies of scale, odour emissions, safety risks associated with combustible matter or gases, and premature wear of the drier components caused by the need to add sand to crumble the waste during treatment. Objectives The Sludge's Wealth project aimed to demonstrate the cost effectiveness of a new sludge waste treatment plant for offshore oil rigs and similar units. The project planned to develop a pilot process based on a hybrid microwave and hot air heating technology with associated equipment to allow the crumbling of semi-dried sludge without any external mechanical action. The project had several specific targets for the pilot treatment unit to meet the characteristics of an oil rig environment and environmental goals around low energy consumption and valorisation of waste: Dimensions of only 12x3x3 metres; Capacity to treat at least 650 t/year of biological sludge – equivalent to 60 000 inhabitants - with humidity at 75%; An energy demand of only 250kW; Generation of 280 t/year of pellets with high calorific content to be valorised as a thickened fuel source.The new system hoped to achieve both: a high efficiency in drying the sludge; and sanitisation of the aggregated end product due to the direct absorption of microwaves by water-containing micro-organisms, such as coliform bacteria. The project thus aimed to provide a realistic and cost-efficient alternative to landfilling sludge waste or inefficient incineration in small capacity units. Results The Sludge’s Wealth project successfully demonstrated a cost-effective and efficient pilot treatment system for relatively small volumes of biological sludge, such as found in an oil-rig context. The 12m-long pilot plant was able to convert biological sludge with 75% humidity into combustible pellets, based on a hybrid microwave and hot air heating technology, showing excellent environment results. The project carried out sludge characterisations and initial tests with different treatment technologies. The project team then provided a full design of the pilot plant. The designs covered all elements including the drying tunnel, the position and number of microwave generators and the electromagnetic field distribution. They also designed a system to level and convey the sludge into the process; however they did not yet include a feeding component to upload the material, which had to be done manually. The team built and assessed a fully operational prototype, continuing to provide improvements and identifying several optimal parameters to ensure efficiency and safety, including a system to cool the magnetrons to avoid breakdowns. The system achieved the following results: Annual conversion of 650 t of organic sludge with 75% humidity; Generation of 280 t of pellets with high heating power and easily transportable; Sterilisation of the agglomerated products through the interaction of the microwaves with micro-organisms containing water; Highly efficient drying of the sludge (14-18% humidity); Elimination of unpleasant odours produced by the material; Energy density of microwave dispersion not greater than 1; Longer life of the system's components due to the lack of abrasion in the process; Reduction of the system’s energy balance by 30% - this energy efficiency decreases if special machinery is used to produce hot air.A key feature of the design was the decision to use a mobile structure that could be easily transported without having to handle and transport all the dangerous sludge waste. This provided for reduced CO2 emissions as well as reduced health and environmental risks. The portability and small size of the plant makes it a very suitable and cost-effective option for small towns and specific contexts such as oil rigs. Furthermore, the good environmental results achieved by the system offer excellent potential to be applied to different types of sludge, thus providing a transferable solution for several sectors. The beneficiary is now defining the processing and operating parameters for the use of the system for the treatment of grape-seed and olive-pomace wastes. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).