Elsevier

Aquacultural Engineering

Volume 23, Issue 4, 1 October 2000, Pages 315-332
Aquacultural Engineering

Reactive nitrogen and phosphorus removal from aquaculture wastewater effluents using polymer hydrogels

https://doi.org/10.1016/S0144-8609(00)00058-3Get rights and content

Abstract

We have developed poly(allyl amine hydrochloride) (PAA · HCl) polymer hydrogels, that efficiently remove nitrate (NO3), nitrite (NO2), and orthophosphate (PO43−) nutrient anions from the aquaculture wastewater. The hydrogels were prepared by chemically crosslinking linear PAA · HCl chains with epichlorohydrin (EPI). The anion binding capacity of the pH sensitive polymer gels was measured in standard solutions and studied as a function of gel synthesis parameters. Equilibrium NO3–N, NO2–N, and PO4–P loading of 15, 1.6, and 17 mg/g of dry gel, respectively, were calculated from the measurement of decrease in anion concentration in aqueous solutions using UV–vis spectrophotometry. Batch experiments showed that nutrient concentrations in aquaculture wastewater effluents decreased with regard to PO4–P by 98+%, NO3–N by 50+% and NO2–N by 85+% within 3 h of reaction. The regeneration of the hydrogels was demonstrated by the release of bound nutrient anions upon washing the gels with a 1 N NaOH solution. These results have demonstrated that the hydrogels are appropriate materials for treating aquaculture wastewater effluents, and reducing the nutrient anion concentrations to levels, less than 10 mg/l NO3–N, 0.08 mg/l NO2–N, and 0.3 mg/l PO4–P, suitable for discharge into natural surface waters.

Introduction

Aquaculture is a rapidly expanding industry that requires quantities of water in the order 200–600 m3/kg fish produced (Sauthier et al., 1998). Many aquaculture production facilities operate as ‘flow through’ or ‘open’ systems, thus releasing large quantities of nutrient rich water into another receiving body of water. Commercial recirculating or closed water systems are being developed to minimize the quantities of water used (Sauthier et al., 1998) and have achieved a certain degree of success. Given the increase in land prices, water shortages, and governmental regulation on the effluents from the aquaculture facilities, closed water systems will be needed if aquaculture is to continue growing (Stickney, 1994). Natural waters can be contaminated, and their quality decreased, by the discharge of nutrient anion pollutants such as nitrate, nitrite and phosphate. Most natural waters are nutrient limited. High NO3, NO2, and PO43− concentrations are found in the wastewater discharge of recirculating and, to some extent, flow through aquaculture production systems.

At sufficiently high concentration levels nutrient anions could become toxic to fish or lead to the crash of a phytoplankton bloom resulting in the rapid growth of filamentous algae or undesirable macrophytes (Ng et al., 1993, Stickney, 1994, Sauthier et al., 1998). Algae blooms, (aquatic plant growth), produce unsightly areas, lower dissolved oxygen (DO) concentrations in the water and may lead to fish mortalities. High levels of nutrient anions lead to eutrophication of receiving water bodies resulting in the creation of marshland (Stickney, 1994). Productivity is the most conspicuous aspect of cultural eutrophication. It is accelerated by the runoff from the aquaculture waste discharges, rich in nutrients.

NO2 is highly toxic to certain species of fish. NO2 enters the bloodstream through the gill by the mechanism that normally transports chloride (Boyd and Tucker, 1998). After entering the bloodstream NO2 oxidizes the iron in the hemoglobin molecule from the ferrous state (Fe2+) to the ferric state (Fe3+). The resulting product, called methemoglobin, is incapable of reversibly binding with oxygen, so exposure to NO2 causes respiratory distress because of the loss in blood oxygen-carrying capacity. Other lesser effects contribute to NO2 toxicity as well. NO3 is the least toxic of the major inorganic nitrogen compounds. However, high levels NO3 can affect osmoregulation and oxygen transport, but toxic concentrations are much higher than for ammonia and NO2 (Zweig et al., 1999). Recommended inorganic nitrogen levels in ponds and tanks on a general basis are <0.15 mg/l for NO2–N and <23 mg/l for NO3–N (Zweig et al., 1999). The recommended release concentrations are <11 mg/l NO3–N, 0.03 mg/l NO2–N, and 2.6 mg/l PO4–P. Interest in the impacts of wastewater discharge from the aquaculture production has not been limited to areas where cold-water facilities were releasing their wastewater into rivers, streams and lakes. Various states as well as the federal government have began developing effluent guidelines (Stickney, 1994).

The removal of dissolved nutrients requires the employment of more advanced technology, than primary and secondary treatment, such as ion exchange resins that will scrub the nutrients from the water. Inorganic and polymeric sorbents, such as clay minerals, zirconia, titania, polymeric ligand exchangers and activated alumina have been investigated as adsorbents of nutrients (especially PO43−) in water (Urano and Tachikawa, 1991, Zhao and Sengupta, 1996, Zhao and Sengupta, 1997). These conventional adsorbents, however, may not be feasible in practical wastewater treatment because their adsorption capacities are insufficient and the processes using these sorbents have not been fully developed. Some of the shortcomings with these sorbents can be summarized as follows:

  • poor selectivity towards one anion over other competing anion species (i.e. poor PO43− binding due to the presence of sulfate (SO42−), chloride (Cl), bicarbonate (CO32−), and dissolved organics);

  • low capacity in the neutral pH range;

  • insufficient regeneration;

  • gradual loss in capacity due to dissolution of the sorbent or fouling by organic matter;

  • long times to achieve desired anion removal.

Polymeric hydrogels are hydrophilic three-dimensional polymer networks that can absorb large amounts of water. The resulting water swollen polymer network is prevented from dissolving because of the presence of tie-points between the polymer chains such as crosslinks, physical chain entanglements, or crystalline regions. Poly(allyl amine), or, its analog HCl form (poly(allyl amine hydrochloride), PAA · HCl), is a water-soluble cationic polymer that can be chemically crosslinked to produce a highly water swollen hydrogel. PAA · HCl is a polymer having pendant primary amino groups (NH2). When the PAA · HCl polymer, is placed in an anion-containing solution, counterions bind through electrostatic association to the pendant to the main chain protonated amine polyions (NH3+) and are trapped into the gel network. Thus PAA · HCl hydrogels selectively bind the nutrient anions into the polymer matrix permitting their subsequent removal from the contaminated system. Previous study performed in the batch mode indicated that the PAA · HCl hydrogels are capable of effectively removing PO43− from the aquaculture wastewater (Kioussis et al., 1999, Kioussis, 1998). The focus of this study was the development of novel polyelectrolyte hydrogel materials for the removal of NO3 and NO2 as well as PO43− from the aquaculture effluents.

The anion binding properties as well as the final microstructure of the PAA · HCl gels are influenced by the relative amounts of materials used in their synthesis. In this study the amounts of the reactants used to synthesize the hydrogel were varied in order to optimize the resulting hydrogel properties. To be effective in removing nutrient anions from the wastewater effluents, the rate of transport of the anions into the gels must be large enough so that efficient binding can be achieved. The transport process must also be reasonably insensitive to pH changes so that the anions can be bound from the aquaculture effluents of varying pH. Furthermore, the presence of particulates (suspended solids), organics, and counterions that are common fouling constituents of aquaculture system effluents should not limit the nutrient removal ability of the gels. The ultimate goal of this research will be to place the PAA · HCl gel into packed columns in commercial aquaculture systems with wastewater effluent being pumped over the column.

Section snippets

Materials

PAA · HCl solid powder, sodium hydroxide (NaOH) (pellets, 97%), epichlorohydrin (EPI) 99+% solution, potassium dihydrogen phosphate (KH2PO4) (99+%), sodium nitrate (NaNO3) (99+%), and sodium nitrite (NaNO2) (97+%) were purchased from the Aldrich Chemical Company. All reagents were ACS grade and were used without further purification. Hybrid striped bass and tilapia fish tank wastewater from the Department of Biological Resources Engineering at the University of Maryland was used. Wastewater

Hydrogel anion binding capacity measurements

The effectiveness of the PAA · HCl hydrogels in removing NO3 and NO2 from the wastewater was initially investigated in NaNO3 and NaNO2 aqueous standard solutions. Fig. 2 shows the NO3–N concentration decrease with time upon the addition of 0.01 g PAA · HCl gel to a batch reactor containing 40-ml NaNO3 solution. The hydrogel reached its saturation point after 2.5 h of reaction time and the NO3–N concentration remained constant at 1.5 mg/l. More than 70% of NO3–N initially presented was

Discussion

It was possible to determine the NO3 and NO2 binding capacity of the PAA · HCl gels and the effect of synthesis parameters on the gel anion removal ability from the studies conducted in controlled environments containing only the target anion (i.e. standard solutions). The ionic strength of the anion standard solutions was determined to influence the anion removal ability of the gels. Higher anion binding capacities were achieved in standard solutions of higher ionic strength.

Experiments

Conclusions

The PAA · HCl hydrogels exhibited efficient nutrient removal in both anion standard solutions and aquaculture wastewater originating from the recirculating aquacultural production systems. The relative amounts of NaOH and EPI used during the synthesis reaction of the PAA · HCl gels were independently varied to determine their possible influence on the anion binding capacity of the gels. The experimental results indicated that the binding capacity of the PAA · HCl gels decreased with increasing

Acknowledgements

This study was supported from the Maryland Agriculture Experiment Station through a competitive research grant.

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