Two major wastewater treatment plants in the metropolitan area of Madrid, Spain – Madrid-Sur and Madrid-Butarque – will meet European Union (EU) regulations for water management and sludge disposal once sludge treatment is centralised and thermal drying facilities are completed, reports Mark Kragting.

Together, the plants’ designed drying capacity will serve a population of four million – equivalent to 36.5 tons of water evaporation per hour and an output of 250 tons of dry product per day. The granulated final product contains less than 10% moisture. The exhaust gas of a co-generation plant is used for dryer heating. The co-generation plant produces 24kW of electric power, while the heat mass of the exhaust gas covers 100% of dryer energy needs. Plants are designed according to strict Vadeb guidelines for Risk and Safety Control to ensure a reliable 8,000 hrs/year operation. These two large wastewater treatment plants in addition to a number of smaller treatment facilities serve the metropolitan area population of some four million people. The two plants serve 4 million population.

Each plant is responsible for its own waste sludge disposal – landfill, land-application and composting. EU regulations require sludge treatment to be centralised, using thermal drying as the key process at the two major treatment sites.

Design, construction and operation of the sludge treatment facilities have been privatised to the Spanish company Sufi SA, based in Madrid. Boremer SA, a joint venture between Sufi SA and civil contractor Cadaqua SA, awarded the turnkey contract for two-line sludge drying facilities to Vandenbroek International for the Madrid-Sur and Madrid Butarque wastewater treatment plants.

Overall sludge production in the Madrid area amounts to approximately 90,000 tons of dry solids a year, which requires a total treatment capacity at the Madrid-Sur and Madrid-Butarque sites of about 10 tons/hr as dry solids, given 35 tons of water evaporation per hour. The sludge types at Sur and Butarque are characterised by two-stage anaerobic digestion of primary and secondary sludge. Upstream of the digestion process, sludge is pre-treated with the use of rotary screens and dissolved-air-flotation. After secondary digestion, belt presses and centrifuges mechanically dewater the sludge. The wet cake has an average dry solid content of 23 %.

The treatment concepts of the Sur and Butarque treatment facilities are similar. Sludge cake from small regional wastewater treatment plants is transported to the cake reception facilities and pumped into

intermediate silos. The cake is then conveyed to two-line sludge dryers.

Both plants generate electrical power with natural gas-fuelled gas turbines (Sur) and gas engines (Butarque). The generator-exhaust gas is the main heat supply for the drying process, covering approximately 100 % of the total dryers’ heat demand.

The rotating drum dryers are of the indirect convective type; the hot exhaust gases of the gas turbines (Sur) or gas engines (Butarque) transfer heat via heat exchangers to the recycled dryer gas. About 10% of the gas volume passing through the drum must be taken out of the system for final treatment, which involves dedusting and chemical scrubbing.

The final product is granulated sludge, classified into two to six millimetres with a dry-solids concentration of more than 90%. Before final storage the sludge granules are cooled down to below 50°C to prevent self-heating during storage time.

The design of further sludge drying plants at wastewater treatment plants in Valladolid and Leon with capacities of 2-3 tons of water evaporation per hour are similar. The Valladolid dryer, installed in November 1999, has been operating continuously within the guaranteed performance criteria, i.e. granule size, moisture content, fuel consumption, reliability.

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