KEY WORDS

Forestry, nursery, constructed wetlands, surface flow, subsurface flow, ammonia, nitrate, phosphate.

INTRODUCTION

A growing concern within the forest nursery industry in British Columbia is the issue of waste nutrient management. Two pieces of environmental legislation affecting the disposal of the effluent includes the federal Fisheries Act and the provincial Code of Agricultural Practice for Waste Management under the Waste Management Act. Under section 36(3) of the Fisheries Act it is an offence to deposit any harmful substance into water frequented by fish, including water that may eventually enter water frequented by fish. Part 5, Section 11 of the provincial Code of Agricultural Practice for Waste Management states that agricultural waste must not be directly discharged into a watercourse or groundwater. Due to the nutrient content of the water discharging from nursery operations, this effluent is considered as an agricultural waste and therefore cannot be discharged untreated to the environment.

The concept of using wetlands to improve water quality originated in Europe in the 1950's when scientists observed an improvement in water quality when municipal sewage was discharged into natural wetlands (Kadlec and Knight, 1996). In the early 1980's the idea of constructing wetlands that are designed to remove specific pollutants and engineered to fit a specific site began to develop into a defined scientific field of research. Since then, the majority of constructed wetland research has been conducted in the southern United States, Europe and Australia covering a wide range of wastewaters including municipal sewage, landfill leachate, acid mine drainage, pulp & paper effluents, urban stormwater and agriculture wastewaters. Over the course of this research two major design have developed: surface flow wetlands and subsurface flow wetlands (Figure 1). Due to costconsiderations and the low organic content of nursery effluents, surface flow wetlands are the preferred design.

Figure 1. Surface and Subsurface Flow Wetland Designs(from Constructed Wetlands for Water Quality Improvement, 1993)

DESIGN AND REMOVAL PROCESSES

Design of a treatment wetland system is dependant upon a number of factors including: volume of wastewater to be discharged; target pollutants to be removed; discharge guidelines; land area available; and, whether wildlife habitat is to be incorporated into the design. Knowledge of the biogeochemical cycles (i.e. the biochemistry and aqueous chemistry) of the target pollutants critical to design. Wetlands remove pollutants from water by a number of physical, chemical and biological processes which include: sedimentation, precipitation, sorption to soil and particles, assimilation into plant tissues, and microbial transformations. These mechanisms are common to all wetland systems; however, variability within the functional components of each system (i.e. water depth, substrates, plants, wastewater characteristics, flow rates, temperature effects etc.) makes it difficult to predict the response of any one system to wastewater application or to transfer results from one geographical area to another. This fact makes it necessary to assess each situation individually and to design each wetland to best suit the needs of the site. Research conducted to date has demonstrated high removal efficiencies for solids (TSS) and dissolved organic matter (BOD₅) in most wetland designs; however, a wide range of results have been reported in the literature for nutrient and metal removal.

The main constituents in nursery effluents that render a health or environmental concern are nitrogen, phosphorus and copper. As nitrate and ammonia nitrogen can be transformed into nitrogen gas it can be permanently removed from the nursery effluents under the correct environmental conditions. Phosphorus and copper, on the other hand, are conservative and must be removed from the sediments by promoting plant uptake, adsorption processes and chemical precipitation reactions which will permanently remove these constituents to the sediment layer in the wetland.

A. Nitrogen Removal

The various forms of nitrogen present in wastewaters pose both human health and environmental concerns. In the form of ammonia and nitrite it is toxic to freshwater aquatic life at low concentrations. When nitrate contaminates drinking water supplies at concentrations of ten milligrams per litre or higher it interferes with oxygen uptake in infants causing "blue baby daisies". Under extreme circumstances brain damage or infant mortality can result.

Nitrogen removal is accomplished through plant uptake and microbial processes. It is the microbial transformations of nitrification and denitrification that are primarily responsible for removal of the excess nitrate and ammonia from the water. Nitrification (Equation 1) is a process by which two separate bacterial species (Nitrobacter and Nitrosomonas) convert ammonia (NH₄) to nitrite (NO₂) and nitrite to nitrate (NO₃), respectively. Oxygen, which enters the wetland water column by diffusion from the atmosphere and radial oxygen loss from the root zone of wetland plants, must be present in the water for these processes to occur. Denitrification is an anaerobic microbial process which reduces nitrate to nitrogen gas (N₂). This is the most desired nitrogen removal process to promote in a treatment wetland receiving nursery effluents. In environments with a readily available carbon source and very low levels of oxygen present a number of bacterial species will utilize nitrate as an energy source reducing the nitrate to nitrogen gas as shown by Equation 2.

b) NO₂^- + 0.5 O₂ ® NO₃^-
  (mediated by Nitrobacter)

Equations 1a & b. Nitrification

NO₃^- + 0.833 CH₃OH ® 0.5 N₂ + 0.833 CO₂
+ 1.167 H₂O + OH^- (organic matter)

Equation 2. Denitrification

By promoting both nitrification and denitrification through areas of marsh, deep open water, cascades and other wetland features, an acceptable reduction of nitrogen can be realized. Figure 2. represents a simplified diagram of the nitrogen biogeochemical cycle in wetlands. wetlands.

Figure 2. The Nitrogen Biogeochemical Cycle (from Treatment Wetlands, 1996)

Almost all freshwater streams, rivers and lakes in British Columbia are nutrient deficient with phosphorus is the limiting nutrient in most instances. This nutrient deficiency is necessary to maintain the water quality required by B.C.'s salmon and trout fishery and provides the clean clear recreational waters for tourism. As very small increases in phosphorus can induce eutrophication, discharging untreated nursery effluents to the environment is a concern.

Unlike nitrogen, phosphorus is a conservative nutrient and cannot be permanently removed from solution by conversion to a gaseous form. It can be removed from the effluent by chemical precipitation, sorption to soil particles and insoluble organic compounds and assimilation into plant tissues. Through these processes phosphate can be permanently deposited to the litter layer of the wetland. Of these processes it is the chemical precipitation which is most promising. Laboratory studies with greenhouse effluents (250 ppm NO₃ and 95 ppm PO₄) have demonstrated greater than 90% removal of soluble phosphate from solution by increasing the pH from 6.5 to 8.0 (Figure 3). As the dominant ionic form of phosphate changes from the monovalent dihydrogen phosphate to the divalent hydrogen phosphate with increasing pH (Figure 4), the soluble phosphate combines with the calcium present in the effluent and precipitates from solution as calcium phosphate. By incorporating pH modification prior to the wetland system an estimated 97% phosphorus reduction could be achieved.

Figure 3.  PO₄ reduction with pH

Figure 4. Dominant phosphate ion with pH

Copper in water is extremely toxic to aquatic biota, even at concentrations as low as two micrograms per litre. Like phosphorus, copper is a conservative element and must be removed from solution by incorporating it into the sediments. This can be effectively accomplished in wetlands due to a number of factors including: the ability of specific wetland plants to assimilate high levels of copper; a high affinity for copper to bind to organic matter; and, the decreasing solubility of copper with increasing pH. In treatment wetlands planted with broadleaf cattails, up to 200 milligrams copper has been assimilated per kilogram plant biomass. With increasing pH and organic matter the percentage of free ionic copper in solution can be reduced by up to 99%. Additionally, organically bound copper has been established to be non-toxic and constructed wetlands designed with peat substrates have demonstrated up to 98% copper removal efficiencies.

CONCLUSION

In light of the federal and provincial waste discharge regulations, constructed wetland systems may offer an ideal wastewater treatment option to the forest nursery industry. Research over the past 16 years has demonstrated constructed wetlands to be effective tools for nutrient management and indicates that they may be more cost-effective than other treatment alternatives. By understanding the biogeochemical cycles of target wastewater constituents and the processes that occur within natural wetland systems, treatment wetlands can be designed to take advantage of specific natural processes which improve water quality.

The issue of winter dormancy for the wetland systems is not a concern for the application of wetland technology to the nursery industry. Periods of high wetland productivity roughly parallel the forest nursery production season with the winter dormancy period coinciding with winter shut-down and periods of low wastewater production.

REFERENCES

British Columbia Ministry of Agriculture, Fisheries and Food (1993). Greenhouse Vegetable Production Guide for Commercial Growers.

Haberl, R. and Perfler, R. (1990). Nutrient removal in a reed bed system. Water, Science and Technology. 23(4):729-737.

Hardgrave, M. and Hufton, C. (1995). Reed-beds for treatment of hydroponic run-off. Project Number PC67, Horticultural Research International.

Johnston, C. A. (1991). Sediment and Nutrient Retention by Freshwater Wetlands: Effects on Surface Water Quality. Critical Reviews in Environmental Control. 21(5,6):491-565.

Kadlec, R.H and Knight, R.L. (1996) Treatment Wetlands. Lewis Publishers, Boca Raton, FL.

Moshiri, G. ed. (1993). Constructed Wetlands for Water Quality Improvement. Lewis Publishers, Boca Raton, FL.

Reed, S.C. and Brown D.S. (1995). Subsurface flow wetlands - A performance evaluation. Water Environment Research. 67(2):244-248.

Rogers, K.H., Breen, P.F. and Chick, A.J. (1991). Nitrogen removal in experimental wetland treatment systems: evidence for the role of aquatic plants. Research Journal of the Water Pollution Control Federation. 63(7):934-941.

Vymazal, J. (1995) Algae and Element Cycling in Wetlands. Lewis Publishers, Boca Raton, FL.

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