Physical and chemical treatment of landfill leachate - recycling of nitrogen

Abstract

This paper presents a new type of plant to treat leachate from a municipal landfill. The plant is situated in Eskilstuna (pop. 90 000) a town-situated approx. 110 km west of Stockholm. The main purpose of the plant is to reduce nitrogen, phosphorus, COD and heavy metals in the leachate from the Lilla Nyby landfill in Eskilstuna. Lilla Nyby landfill is a municipal landfill, which has been used for nearly 50 years to deposit industrial and municipal waste from the Eskilstuna community. A drainage system in the landfill collects the leachate which is stored/equalized in a basin with a maximum volume of 40 000 m3 . The yearly amount of leachate is about 120-160 000 m3 • The leachate is pumped to the treatment plant. The demands of the authority to meet are as follows: NH3 (ammonium nitrogen) < 25 mg/I COD < 200 mg/I Total phosphorus (tot-P}< 0,3 mg/I The normal composition of the untreated leachate is NH3 (ammonium nitrogen)e- 250 mg/I COD - 400 mg/I Total phosphorus (tot-P)- 1,0 mg/I Thus it is necessary to reduce the ammonium nitrogen with -90% and COD with -50%. A request of a total tender with function warranty was made 2001. 6 contractors offer tender, 3 tenders are judged to be interesting, Pronea, Lackeby and Stork Engineering. Pronea offers chemical precipitation+ammonia-stripping/ absorption+activated coal-filtration, Lackeby offers SBR-treatment alt. NF+ ammoniastripping/ absorption and Stork offers evaporation or reversed osmosis RO). After a technical/economical evaluation the Pronea offer was considered to be the most interesting and was purchased. The process produces a useful chemical, ammonium sulphate, which can be used as an additive to the bio fuelled (wood chips) boiler in Eskilstuna. Two useful results will be reached: l) The corrosion in the furnace caused by alkali metals will be reduced and 2) the ammonia additive to reduce NOX in the flue gases can be reduced. The plant works as follows: 3 • Leachate is pumped to the plant (dim. capacity 20 m /h) • Reduction of pH to 3 with sulphur acid and CO2-stripping (reduce the buffer capacity) • Rise of pH to 11,0 with sodium hydroxide and precipitation with ferric chloride (FeCb). Dosage of polymers to enhance the build up of flocks. • Flock separation in lamella separator • Further flock separation in continuous sand filter • Rise of temp. to 50-60 0C through heat exchange • Ammonia-stripping + ammonia-absorption • Filtration in activate coal filter+ pH-adjustment Nitrogen in the leachate (ammonium nitrogen) together with sulphuric acid forms ammonium sulphate, at a concentration of 40% The plant is not yet approved and taken over by the purchaser. Much work has been needed to trim the process in the plant. A lot of problems have appeared and been solved. Some examples of the most essential items: • Foam forming after the CO2-stripper • Transport of water drops from the ammonia-stripper to the ammonia-absorber • Break down of process pumps for leachate (carbonate deposits), dosage pumps and valves • Leakage of sulphuric acid and sodium hydroxide • Sludge escape from the lamella separator (bad separation) • Clogging in heat exchangers and stripper tower(carbonate deposits) • Wrong placed flow meters Preliminary results are: • At least 90% ammonia reduction is possible • Good phosphorus- and metal reduction • If approved COD-reduction can be reached is still to be proved • Good quality of produced ammonium sulphate • The need of chemicals and energy yet to be evaluated