Abstract.The paper deals with recovery of phosphorus from the liquid discharge from dewatered wastewater treatment sludge via electrodialysis. Using electrodialysis, phosphorus was transferred into concentrate, where phosphorus concentrated to673.6 mg/l (from the original value 79.87 mg/l). This enriched concentrate may be further processed, e.g. phosphorus can precipitate into the form of struvite.
Sludge water, a product of drainage of primary and activated sludge from the municipal wastewater treatment process, is rich innutrients, particularly nitrogen and phosphorus. Nitrogen and phosphorus are inorganic biogenic substances suitable for plantnutrition. Sludge water can be treated via electrodialysis. This process leads to two products, one is a diluate stream (solutionless concentrated than the input, contains some nutrients) and the other is concentrate stream (solution in which nutrients areconcentrated against the input). There is abundant research in nitrogen and phosphorus recovery from the concentrate stream, forexample in the form of struvite used for fertilizing. However, little attention has been paid to the huge volumes of diluate stream.The diluate stream can be used in agriculture as water suitable for irrigation, where the bonus of this product is a partial fertilizingcapacity. This way, wastewater can be used for irrigation and fertilizing having subjected the wastewater to agrochemical analyses.An important condition for the use of sludge water or diluate stream for irrigation is the absence of heavy metals and toxic substances.Still, if the water contains heavy metals, they can be removed with a relatively high success, for example by precipitation.Using wastewaster as water for agricultural purposes for irrigation is one of the new options to find new secondary sources of water.This theme is highly topical as it is important to reduce the consumption of quality water resources. These must primarily serve fordrinking purposes, and therefore it is necessary to search for alternative water resources.
Digestate from biogas plants, formed by dewatering anaerobically stabilized sludge, is characteristic of high concentrations of phosphates and ammonia nitrogen suitable for further use. Phosphorus is an element widely used to produce fertilizers, and because of its continually shortening natural supplies, recycling of phosphorus is gaining on significance. Both phosphorus and nitrogen are important elements and their presence affect the quality of water resources. Both elements can contribute to eutrophication. At the same time, both phosphorus and ammonia nitrogen, are important elements for agricultural production, and therefore greater demands are being made on the effort to connect sewage treatment processes and the process of recycling of these nutrients. A suitable product of phosphorus and ammonia nitrogen are phosphates in the form of a structurally-poorly soluble precipitate of magnesium ammonium phosphate (struvite). This form of slowly decomposing fertilizer is distinguished by its fertilizing abilities. Compared to direct use of digestate as a fertilizer, struvite is more stable and can gradually release ammonia nitrogen for a long time without unnecessary losses. In the reported experiments, the precipitation efficiency of the recycling of ammonia nitrogen and phosphorus from the digestate liqour (liquid discharge from digestate) was, at a stoichiometric ratio of Mg2+: NH4+: PO43− (3.2: 1: 0.8) and a stirring time of 15 minutes, 87 % for ammonia nitrogen ions.
The need to treat wastewater in a sustainable way to minimalize contamination and maximize the recovery of nutrients is widelyrecognized. The focus is mainly on the removal and recovery of nutrients because of eutrophication problems in receiving waters,limitations of mining resources and high costs affiliated with nutrient production. Removal of nutrients is also a growing problemfor water authorities, as authorities often tighten standards of discharged waters to avoid unnecessary discharge of nutrients intowater bodies.The universally used technologies for nitrogen and phosphorus removal involve biological nitrification and denitrification and metalsalt precipitation. However, applying these technologies nutrients are made unrecoverable for fertilizing. Electrodialysis (ED) isa membrane process capable of concentrating and separating ions from wastewater. Thanks to the applied current, the migration ofions occurs, and ions are concentrated in the concentrate solution. Laboratory scale ED showed the potential of nutrient recoveryfrom wastewater sources.In this study, nutrients were recovered in concentrate solution from wastewater sludge liquid discharge by using electrodialysis.Calcium and sodium values in concentrated solution increased 10 times, values of potassium and chloride 5 times. The amount ofammonia nitrogen raised from 60 mg/l to 1700 mg/l. The concentrate enriched by nutrients may be used in further processes, e.g.phosphorus and ammonia nitrogen can be precipitated into the form of struvite.
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