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Recovery of Nutrients from Wastewater via Electrodialysis

The need to treat wastewater in a sustainable way to minimalize contamination and maximize the recovery of nutrients is widely recognized. 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 problem for water authorities, as authorities often tighten standards of discharged waters to avoid unnecessary discharge of nutrients into water bodies. The universally used technologies for nitrogen and phosphorus removal involve biological nitrification and denitrification and metal salt precipitation. However, applying these technologies nutrients are made unrecoverable for fertilizing. Electrodialysis (ED) is a membrane process capable of concentrating and separating ions from wastewater. Thanks to the applied current, the migration of ions occurs, and ions are concentrated in the concentrate solution. Laboratory scale ED showed the potential of nutrient recovery from 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 of ammonia 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.


Introduction
The biologically treated secondary effluent from existing wastewater treatment plants still contain a certain amount of nutrients (such as NO3 -and HPO4 2-) (Zuo, 2014) and salinity (such as Na + , Ca 2+ , K + , Mg 2+ and Cl -). The insufficient treatment could lead to eutrophication (Wang, 2015) and influence the reuse of reclaimed water (Liang, 2013). Moreover, phosphorous is an important mineable resource (Wang, 2013), and recovering phosphorus (P) from wastewater streams for reuse can alleviate the problem of limited phosphate resource supply (O'Neal, 2013). N and P contents in wastewater and surface water are increasing, while the reservation resources of them are being depleted, especially for phosphorus. N and P may thus be recovered from wastewater or surface water. After making a general survey of activated sludge process, interests in recovering nitrogen and phosphorus focus on the excess sludge developed based on some additional reforming operation and newly introduced technologies (Tong, 2009;Wang, 2013). Anaerobic digestion process can achieve the degradation of organic matters and the formation of biogas, and it has been widely used to treat the excess sludge. Sidestreams, or "return liquors", typically have a very high concentration of ammonia and phosphorus because they are released in the anaerobic digestion process: Ammonia is produced by the biological degradation of the nitrogenous matter, such as bacteria, proteins and urea; Phosphorus is produced by re-solution of the part incorporated into bacterial biomass in the enhanced biological phosphorus removal (EBPR) process (Münch, 2001). It is unwise to return the solution rich in ammonia and phosphorus back to the sewage treatment plant, because the containing nutrients will be aggregated in the newly produced excess sludge rather than through decontamination.
Recently, membrane filtration technologies, such as reserve osmosis (RO), nanofiltration (NF) and electrodialysis (ED), have drawn much attention to desalinize and recover useful materials in wastewater treatment (Werner, 2013). Pressure membrane processes, such as RO and NF, need a quite complex pretreatment to alleviate the membrane fouling (Jamaly, 2014; Byun, 2015), resulting in needless capital cost and energy consumption. Furthermore, RO is ion uncontrollable and highly relying on the size of molecules (Kwak, 2013). However, as an electrochemical membrane separation process, ED is capable of separating undesired ions from wastewaters more energy-efficient (Vaselbehagh, 2015) with an applied electric field as the driving force to get a higher water recovery (Kwak, 2013).

Materials and Methods
Feed water for this experiment consisted of wastewater sludge liquid discharge. The samples of wastewater were recovered three times over period of six weeks. Each sample was analyzed and run through electrodialysis (ED). The chemical analysis of the feed water used in this experiment is shown in Table 1. The feed water was pre-treated by vacuum filtration.
Feed water was also analyzed by atomic absorption spectroscopy (AAS) to find out if the samples contained heavy metals. The AAS analysis is shown in Table 2.
A laboratory-scale electrodialysis module ED(R)-Z setup was used in this study, and it was composed of electrode compartments, concentrated compartments, and diluted compartments. Compartments were separated by cation/anion exchange membranes, silicone gaskets and plastic partition nets. In addition, the specific parameters are summarized in Table  3. The membranes used in electrodialysis included cation exchange membranes and anion exchange membranes.

Result and Discussion
Concentrated solution from electrodialysis was obtained in total volume 0.5 l per sample and was analyzed, along with dilute (2 l per sample), in laboratory and by AAS. Samples of concentrated solutions were kept in a dark and cold place. Analysis was conducted the next day in the laboratory. Results of the concentrated solution analysis are shown in Table 4.
Samples were preserved with nitric acid and analyzed by AAS. Results of the concentrated solution analysis are shown in Table 5.
As it is clear from the presented results, the ED process was successful in concentrating elements, such as ammonia nitrogen, chloride, calcium, potassium and sodium. These elements, along with phosphorus, are essential plant macronutrients.
K can occur in unavailable, slowly available, or readily available forms in soils. Only a small amount of slowly avail-able K is available for plant uptake during a single growing season. Readily available K is dissolved in the soil solution and readily taken up by plants. Potassium uptake is affected by soil moisture content, soil aeration and oxygen level, soil temperature, and competing ions.
Like K, Ca and Mg in soils originate from decomposition of bedrock and minerals that contain these elements. Compared to other minerals, Ca weathers relatively quickly and can become unavailable to plants via leaching in highly weathered (mature) soils.
Sodium is not an essential element for plants but can be used in small quantities, similar to micronutrients, to aid in metabolism and synthesis of chlorophyll. In some plants, it can be used as a partial replacement for potassium and aids in the opening and closing of stomates, which helps regulate internal water balance. Chloride is needed in small quantities and aids in plant metabolism, photosynthesis, osmosis (movement of water in and out of plant cells) and ionic balance within the cell.

Conclusion
The results demonstrate that a concentrated product could be achieved by concentrating nutrient ions from wastewater sludge liquid discharge to concentrate product stream via the ED process. The concentrated solution contains ions in a soluble form, therefore there is a need to continue this research and precipitate the said ions into forms that may be used as fertilizers.
Ammonia nitrogen and phosphorus could be precipitated into the form of magnesium ammonium phosphate, which is known to be an ideal alternative fertilizer since it is a non-odorous, non-sludgy crystal which releases nutrients slowly.