Issue 62: Passive Wetland System for Intensive Horticulture

Issue 62
January/February – 2002
Story Title: Passive Wetland System for Intensive Horticulture
Author: Steven Carruthers

STEVEN CARRUTHERS writes that the integration of a passive wetland system to remove sediments and nutrients from waste water, points the way for the hydroponics and greenhouse industry to show its environmental credentials.

The passive wetland system is an effective way to remove sediments and nutrients from waste water.

The Natural Heritage Trust, in collaboration with NSW Agriculture, has funded a two-year project to develop a closed, passive wetland system to manage nutrient-loaded waste water. According to NSW Agriculture Horticulturist, Jeremy Badgery-Parker, the wetland model located at the National Centre for Greenhouse Horticulture, Gosford, demonstrates to growers how they can minimise sediment and waste water impacts on the environment from intensive farming practices.

“Every farmer faces a range of issues, problems and restrictions in their operation,” said Badgery-Parker. “This model provides an insight into a flexible and efficient way of removing nutrients and sediments from run-off water.”

For hydroponics and greenhouse growers, the integration of a passive wetland system on the farm is the final piece in the environmental jigsaw that will show their operation, and the industry, as environmentally sustainable.

Is there waste water from hydroponics?
Waste water or ‘run-off’ from intensive horticulture usually contains high levels of nutrients, and possibly some sediments and plant pathogens. Described as nutrient-loaded, this waste water is defined as effluent.

In hydroponics there are two types of waste water – run-to-waste and dumped. In run-to-waste, nutrient-loaded water comes from flow-through hydroponic systems that use a growing medium. While it is possible to have no excess water from these systems, most growers will have waste water because it makes the management of plants and nutrients easier. For example, applying more water than the plant uses can prevent the build-up of salts around plant roots by washing them out of the growing medium.

In the second type of waste water, residual nutrient solution in recirculating systems is periodically dumped, or when a nutritional or disease problem arises. In this situation, discarding the nutrient solution and starting again is often the most economic option. However, it’s these types of practices in hydroponics and greenhouse operations that harm the industry’s reputation as environmentally sustainable.

The wetland species Tussock Rush (Juncus usitatus) is indigenous to the NSW Central Coast region.

What to do with waste water
The wetland species Tussock Rush (Juncus usitatus) is indigenous to the NSW Central Coast region.

Before the hydroponics and greenhouse industry can claim it is environmentally sustainable, nutrient-loaded waste water must be cleaned prior to its discharge into the environment. A number of methods can be used to stop nutrients and sediments being carried into water courses including bio-filtration (wetlands), filtration, evaporation and reverse osmosis. Of these, an on-farm wetland system is the simplest and most economical way for growers to clean their waste water. The advantages of hydroponics are that the nutrient solution is a known quantity and concentration, and it can be readily collected from the hydroponics system.

The two key nutrient pollutants found in waste water are phosphates and nitrates. In natural water systems, total nitrogen levels of 0.1-0.75mg/L contribute to algal blooms, but the level of phosphorus in water is often the key factor causing algal blooms. Total phosphorus levels of 0.01-0.1mg/L contribute to algal blooms.

Phosphates tend to attach to sediments, particularly clay particles, which can be removed through sedimentation. Nitrates are highly soluble compounds which can be removed by plant uptake and biological conversion to nitrogen gas. In wetlands, conversion to nitrogen gas by micro-organisms is the main process of removal, where the nitrogen is released into the atmosphere. Data suggests that micro-organisms will convert up to 90% of nitrates to nitrogen in a passive wetland system in two days.

Nitrogen is also taken up by wetland plants. The organic matter produced by plants can be used as a mop to pull some of the nutrients out of the waste water. Weeding, mowing and otherwise removing plant material from the wetland then removes nutrients from the system.

“Green waste is much easier to manage than invisible nutrients dissolved in water,” said Badgery-Parker. “The plant materials can be composted, used in the garden or orchard, or disposed of through local green waste collection services.”

NSW Agricultural Horticulturist Jeremy Badgery-Parker. The gravel-based artificial wetlands offer more surface area for micro-organisms to do their job converting nitrates into nitrogen gas.

Passive waste water treatment
NSW Agricultural Horticulturist Jeremy Badgery-Parker. The gravel-based artificial wetlands offer more surface area for micro-organisms to do their job converting nitrates into nitrogen gas.

The first step is to contain waste water on-farm so that the nutrients and sediments can be removed from the water. An effective passive system can continually clean waste water with only minor maintenance and minimal running costs. The wetland system only needs periodic removal of sediment and vegetation.

A wetland water treatment system uses lessons learned from nature. In normal natural processes, run-off water carries sediments, nutrients and plant material into water courses. These natural contaminants are removed by micro-organisms, plants and animals downstream. Intensive farming, along with other activities such as residential developments, can lead to higher levels of nutrients entering the water system. As a result, sometimes the natural system can not cope.

The wetland is nature’s filter and has been shown to be effective in removing nutrients and sediments from water. One option for the responsible farmer is to use a passive wetland system to remove sediments and nutrients before the water leaves the farm.

“The use of wetlands has increased in recent years as an effective and friendly method of treating waste water,” said Badgery-Parker. “The objective is to construct a simple wetland system that is effective and feasible for the small intensive farm. However, even wetlands need maintenance, so they need to be constructed with this in mind.”

A workable wetland for intensive farms
With funding from the Natural Heritage Trust, NSW Agriculture has designed a closed, modular wetland system using some of the principles of natural wetlands. Mr Jeremy Badgery-Parker said that a feasible system must be compact, easy to build, and flexible enough to incorporate existing components that might be found on the farm.

The four basic components of the closed wetland system are sediment trap, soil and grass soak bed, wetland area, and retention pond. The modular wetland system at the National Centre for Greenhouse Horticulture embraces all these components designed to remove sediments, nitrates and phosphates over a five-day treatment period, based on draining 1,000 litres of waste water from the hydroponics system each day. In this demonstration project, inspection points are located at the inlets and outlets of each module where the water can be tested and analysed for nutrient load.

A guidebook (in press) is available from NSW Agriculture which contains more information on how to go about building a wetland system to deal with larger volumes of waste water.

The function of the sediment trap is to settle out heavy sediments, floating litter, and phosphates. Sand and lime assits in the removal of phosphates.

Sediment trap
The function of the sediment trap is to settle out heavy sediments, floating litter, and phosphates. Sand and lime assits in the removal of phosphates.

The first module of the closed wetland system is a sediment trap to remove heavy sediments, phosphates, and litter such as root debris, perlite or other growing media. Once nutrient-loaded waste water drains into the trap, the sediments settle out of the water and are left behind when the water moves on.

“The sediment trap does two things,” said Jeremy Badgery-Parker. “First, it slows and stops the waste water. As soon as you stop the water, solids tend to fall out of solution or they float.

“Then the water is drained from the sediment trap, with floating objects caught by a sock over the outlet. Because of this containment, solids and litter can be disposed of properly rather than letting them go down the creek or water course and causing problems,” he said.

Jeremy added that when sediments fall out of waste water, particularly if it’s soil sediment, a lot of pathogens may also go with it. Data suggests that Phytopthora, a common plant pathogen, is removed in the wetland system.

For hydroponics and greenhouse growers, nutrients are the main issue. Because phosphates tend to attach themselves to sediments, some of the phosphorus is also removed simply by capturing the sediment. However, intensive farming systems often have very high phosphate levels in the run-off. To accommodate this, the sediment trap has been modified by adding lime (calcium carbonate) enriched sand to act as a strong phosphate filter.

“Lime readily combines with phosphate to form calcium phosphate, which precipitates out of solution,” said Badgery-Parker.

The soil-based, vegetated sok bed is separated from the ground by a plastic liner. This module removes fine sediments and the primary nitrate load.

Soil and grass filter bed
The soil-based, vegetated sok bed is separated from the ground by a plastic liner. This module removes fine sediments and the primary nitrate load.

The second module is a soil-based grassy area located downhill from the sediment trap, and used as a natural soak bed to remove fine sediments and the primary nitrate load. To ensure no nutrients are leached to the water table, the module is separated from the ground by a watertight plastic liner.

“Grass is an effective user of nitrogen and is also easy to maintain, making it useful for the filter bed area,” said Badgery-Parker. “By removing grass clippings, nutrients are taken out of the system.”

Jeremy said that most grasses do fairly well in wetlands and boggy areas, although growers should be cautious of weed species.

“Apart from removing fine sediments from the waste water, and the grass consuming nitrates, the anaerobic environment converts nitrates to nitrogen gas,” he added.

If growers have got the space, irrigating a grassy paddock with run-off water will do the same job. However, in a closed wetland system, growers have total control by preventing any leakage to ground water.

“The whole focus here, is it’s got to be cheap,” said Badgery-Parker. “We can’t expect growers to build a greenhouse, then meet the extra expensive of a water treatment system.”

Again, the outlet from this module is covered with a sock to prevent fine sediments leaving the filter bed. The water also has less nitrates than when it entered the soak area.

The soil-based “constructed wetlands gives the waste water a final polish to remove the fine sediments and nitrogen.

Wetlands
The soil-based “constructed wetlands gives the waste water a final polish to remove the fine sediments and nitrogen.

The wetland stage of the system gives water a final ‘polish’ by removing any remaining sediments and nutrients.

“The sediment trap takes out a lot of sediments and phosphates, the grassy filter bed removes a lot of nitrates, and the wetland system polishes the water by removing any remaining sediments and nitrates,” said Badgery-Parker.

In this demonstration system, there are two types of wetlands, that Jeremy defines as ‘constructed’ and ‘artificial’ wetlands. The constructed wetland is a soil-based vegetated area that basically mimics a natural wetland system, while the artificial wetland uses a vegetated gravel bed through which water flows. Both modules are 60cm deep and separated from the ground by a plastic liner.

In the constructed wetland, the module is back-filled with soil to a depth of 20cm, and planted with local indigenous plants. The inlet pipe has holes along its length and is positioned across the width of the module in order to spread the water evenly across the wetland. The spreader device allows the water to move slowly and evenly through the wetland system, leaving behind fine sediments and allowing plants and micro-organisms to do their job.

The artificial wetland is slightly larger than the constructed wetland, to compensate for the volume of gravel in the module. The guidebook gives calculations for different pond sizes and their water-holding capacity. The pond needs to be large enough to hold three-days worth of waste water.

“There is a lot of thought that needs to go into constructing a wetland pond,” said Badgery-Parker. “Deep ponds tend to be more anaerobic because there is less air at the bottom of the system, and shallow ponds are more aerobic. Deep water also slows water down compared to shallow ponds. Wide ponds also slow down water. There are also temperature effects.”

Compared to soil-based wetlands, gravel-based ponds offer more surface area for micro-organisms that are doing 70% of the job to remove nitrates.

The retention pond can be a deep water section of a wetland pond or a tank.

Retention pond
The retention pond can be a deep water section of a wetland pond or a tank.

The final module is the retention pond, a deep holding area for cleaned water before its disposal into the environment, or re-use. This component may be either a deep water section of a constructed wetland, or a separate pond. A tank can also be used. Once the cleaned water is drained into the retention pond, the one-day retention period is a final opportunity for any remaining sediments to fall out of solution.

The inlet spreader device is designed to slow and spread the waste water evenly as it filters through the constructed wetland.

How big must the system be?
The inlet spreader device is designed to slow and spread the waste water evenly as it filters through the constructed wetland.

The size of the system is based on the desired retention time for the water in the system. That is, the time it takes from when the water enters the system to when it exits. The retention time depends on the ‘nutrient load’ of the waste water. The nutrient load is a factor of both the ‘volume of water’ and the ‘concentration’ of nutrients in the waste water. A system with a five-day retention period will have a total water volume five times the amount of water flowing into the system each day. Data suggests that, given a nutrient load of 250g of nitrogen (nitrate) entering a system per day, a two-day retention period will result in the removal of 90% of the nitrogen.

Although the demonstration system at the Centre is designed for a five-day retention period, the duplication of constructed and artificial wetlands makes it closer to an eight-day waste water retention system.

“This system encompasses all the options available for an efficient wetland system, and growers can adapt different modules for their individual needs,” said Jeremy Badgery-Parker.

To keep costs down, the use of an existing on-farm dam as a wetland area, or irrigating a grassy area with waste water to remove nitrogen, are viable options that will support the grower’s environmental credentials.

Maintenance
The concentration of nutrients in the effluent should be checked when planning the maintenance programme. Maintenance is an important task and it must not be overlooked. In a wetland treatment system, one problem which can occur over time is that the system can become saturated with nutrients. This is like a filter which eventually clogs up if not cleaned. There are two parts to this problem:

Net nutrient gain
Net nutrient gain can occur. The system works by acting as a nutrient sink. That is, it uses more nutrients than it releases. Over time the plant material in the wetland system builds to a point where it actually contributes to the nutrient load in the water. While this is a natural result of a biological system, it is unwanted for obvious reasons. Maintenance of the system requires removal of plant material so that the system continues to use nutrients by growing more plants.

Phosphorus saturation
Phosphorus tends to stick to sediments such as clay particles, but sediment can only hold a limited amount of phosphorus. Eventually, the system becomes full and no more phosphorus can be removed.

The sediment trap could use sand and lime to increase the capture of phosphates. From time to time this sand is replaced to remove the phosphorus from the system.

Fencing the retention pond
Another issue to consider is fencing the retention pond. Some councils may require growers to fence the pond or dam. Check with your local council. For smaller systems, an alternative may be to install a child-proof tank as the retention pond.

Re-using water
Once you have a closed wetland in place, you will no doubt start to think about re-using the clean water, especially as water is becoming more expensive. However, re-use of water may still require more treatment. Plant pathogens may survive in the system, though recent data has shown that Phytopthora is removed in a wetland system. If water needs to be disinfected, there are a number of treatments available including heat, chlorination, ultraviolet radiation and ozone.

Final remarks
With Quality Assurance trending towards environmental management systems, the integration of a passive wetland system to remove sediments and nutrients from waste water is essential for the industry to show it is environmentally sustainable. In a sense, a passive wetland system is like a chemical record. In the event of an environmental mishap, the grower will be able to show he or she has been environmentally responsible.

Although the closed wetland system has been designed specifically with hydroponics and greenhouse growers in mind, market gardeners and other soil growers as well as the aquaculture industry can also benefit from on-farm wetland systems.

“The problem of removing sediments and nutrients has already been solved; it only needs to be applied,” concluded Mr Badgery-Parker.

Further Reading
Badgery-Parker, Jeremy,
Managing waste water from intensive horticulture: a wetland system
Agnote DPI-381, 1st edition, September 2001,
(Protected Cropping), NSW Agriculture, Gosford.

Badgery-Parker, Jeremy,
Managing Waste Water with a Wetland – a guidebook
(Protected Cropping), NSW Agriculture, Gosford.

Sainty, G.R. and Jacobs, S.W.
Waterplants of New South Wales,
(1981): Water Resources Commission, NSW.

Sainty, G.R. and Jacobs, S.W
Waterplants in Australia – A Field Guide,
(1987): Sainty & Associates, Darlinghurst, NSW.

Website Resources
NSW Agriculture Publications
www.agric.nsw.gov.au/publications

Australian Wetlands plant sales
www.wetlands.com.au  Ω

PH&G Jan-Feb 2002 / Issue 62


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