Sustainable treatment of reject water and industrial effluent by producing valuable by-products
Driessen, W.J.B.M.1, Abma, W.R.1, Van Zessen, E.1, Reitsma, G.1 and Haarhuis,R.2, 1Paques BV, The Netherlands, 2Waterstromen BV, The Netherlands
(free)
The sewage treatment plant of Olburgen in The Netherlands has been upgraded in order reach compliance with the European Frame Work Directive for which the discharge of N and P had to be reduced to 10 mg N/l and 1 mg P/l. The Olburgen wastewater treatment plant treats a combination of municipal sewage and industrial effluents. The Olburgen has been upgraded to improve the effluent quality by implementing a separate and dedicated treatment for the reject water from dewatering of anaerobically digested sewage sludge (biosolids) in combination with industrial (potato) wastewater.
The separate treatment of the combined flows of sewage sludge reject water and of industrial wastewater from a potato processing plant proved to be more cost-efficient and area and energy efficient than a combined traditional treatment process. At Olburgen the excess sewage sludge is anaerobically digested and subsequently dewatered. The industrial effluent is derived from a nearby potato processing plant is treated in an anaerobic UASB reactor.
The biogas produced is treated in a biological Thiopaq® scrubber removing H2S to levels from an average of 5200 ppmv to less than an average of 100 ppmv. The Thiopaq scrubber process is characterized by recovering the alkalinity produced in de biological process achieving reduced caustic consumption up to 95 % as compared to traditional chemical caustic scrubbers. The clean biogas is utilized in a CHP unit producing an average of 390 kWh of electrical power.
Reject water from the sludge dewatering of the anaerobically digested sewage sludge and anaerobically treated industrial effluent are both characterized by relatively high concentrations of phosphorous and nitrogen (especially ammonia). In order to remove phosphorous the combined effluents are treated in a so-called Phospaq struvite reactor in which by addition of magnesium oxide struvite (magnesium-ammonium-phosphate) is produced. The produced struvite has been tested and found suitable as substitute for commercial fertilizers. The produced struvite is in compliance to EU regulations. The struvite is harvested at a dry matter of around 50 % and as a valuable by-product intended for use as a slow-release fertilizer.
In order to meet the stringent discharge limits the effluent from the struvite reactor is subsequently treated in a one-step Anammox® reactor to remove residual nitrogen. In contrast to conventional nitrification-denitrification the conversion of ammonium via the anammox (anaerobic-ammonia-oxidation) process does not require organic carbon and uses about 50 %less energy. Therefore a bypass of raw effluent, providing COD as is required for conventional denitrification, is not needed. The use of the anammox process allows valorization of the complete effluent generating maximum possible amounts of biogas.
An overall schematic of the process in shown in figure 2:
