Issue 18: Nutrient Scrubbers

Issue 18
September/October – 1994
Story Title: Nutrient Scrubbers
Author: Steven Carruthers

The proper disposal of large volumes of waste nutrient solution may soon become a major dilemma for commercial hydroponic growers. Steven Carruthers reports on the Algal Turf Scrubber, which may in the future provide an alternative to dumping.

As we approach the 21st century, growers of all persuasions will face the reality that they will need to pass an environmental audit, and for hydroponic growers that means they won’t be able to discard spent nutrients into the soil less its elemental composition meets the of various State and Federal regulations. To with these reductions, growers will need to practice tighter management policies, such as an increase in nutrient efficiency, system efficiency, and a safe means to dispose of spent nutrient solution so that it doesn’t pose a threat to the environment.

With societal attitudes demanding the conservation and preservation of natural resources, soilless culture may well become the preferred method of growing vegetables, fruit and flowers in Australia if the lessons of Europe, and particularly Holland, are anything to go by. By the year 2000, primary producers will not be cable to afford a laidback attitude towards environmental issues without incur ring a significant consumer backlash.

Improving Nutrient Efficiency
The first step to improving nutrient efficiency is regular nutrient solution analysis in order to gain the maximum life and benefit from individual nutrients. Regular analysis also confirms that the crop nutrient requirement is being satisfied, and reduces the misuse and cost of nutrients through improved crop quality and decreased nutritional disorders.

More importantly, a more efficient use of the nutrient solution means less dumpings per year, and it is here where the hydroponicist is the most vulnerable when it comes to responsible management practices. All recirculating systems at some point need to dump spent nutrients, with run-to-waste systems dumping nutrient solutions daily. It is these practices which are the most visible, and most harmful to the environment.

Algal Blooms
The dumping of spent nutrient solutions may lead to or contribute towards outbreaks of blue-green algal blooms (cyanobacteria), which appear in our inland waters, estuaries and the sea from time to time. Traditional broad-acre agriculture also uses chemical fertilisers that contain high ly soluble and concentrated plant nutrients. The concern is that repeated applications of such fertilisers, and in the case of hydroponic growers, frequent dumping, may have long-term, adverse impacts on soil and water quality, the nutrient sources eventually leaching into our waterways and contributing blue-green algae blooms. Sometimes these algae may release toxins into the water which can kill animals and cause illness in human beings. They may also release compounds which import musty or earthy tastes and odours to the water. In still water such as lakes, ponds, dams and reservoirs, these algae may multiply sufficiently in summer months to discolour the water so that it appears green, blue-green or greenish-brown.

These concerns have prompted a re-evaluation of our current agricultural practices, so much so that the reduce in catchments and streams has of agricultural research organisations and regulators to improve water quality.

Algal Turf System
They say that within every problem lies the solution, and it may well be that relatively new technology in the aquarium environment may provide a long-term solution for the hydroponic grower, to eliminate the need to dump spent nutrient solution.

Many marine and freshwater aquariums cannot use water from the surrounding waterways because of heavy metal contamination, high nutrient levels (nitrogen and phosphorus), fertiliser run-off and siltation which can kill aquatic life. These aquariums traditionally use water from the local city supply with sea salt (for marine aquariums) and chemicals added to keep the animals alive. Then, as much as 50% of the water is replaced at regular intervals with new (lower nutrient) water.

But often the city water supply itself may not be free of nutrients. Barrier or mechanical filtration filters and diatomaceous earth filters used by water supply authorities, and chloreine, but they still cannot remove dissolved chemicals from water. To reduce the amount of nutrients in the system, aquarist will often use protein skimmers, ozone and water dilutions.

Now there has been a breakthrough in filtering or removing nutrients from aquarium water, which may well be applied to the hydroponic environment.

In the 1970s, Dr Walter Adey, Curato of Coralline Algae and Director of the Marine Systems Labaratory at the Smithsonian Institution in Washington DC (USA), began studying corals which he kept in small aquariuns around the windows of his laboratory. This led to the development of a more suitable habitat which had most of the natural components necessary to support the typical community structure of a tropical coral reef – high light intensities, wave action and the growth of algae. However once again the fundamental question arose of how to remove the ever-increasing animal wastes (nutrients) in a closed system.

Dr Adey looked to the natural processes for the solution.

Coral reefs have long been recognised as one of the most productive areas of any ecosystem on the earth. The basic underlying principle of this productivity is the presence of reef plants (algae) occurring, inconspicuously, as pavements, low mats or turfs, boring filaments in the reef matrix or symbionts within animal tissue.

The Algal Turf Scrubber
Algae, like other plants, must absorb nitrogen and other Nutrients into their body tissues for growth and reproduction. The development of the Algal Turf Scrubber simply simulates the processes that occur in nature on the weather side of coral reefs as the waves break against the reef and cross the reef top. In this area of turbulent mixing behind the breakers, a zone of short, dense, actively growing algae develops.

The underlying principle behind the growth of turf algae is the necessity for grazing by members of the animal population. The algae must be cut back or grazed, much like the grass in your backyard; or other species which are not as efficient cat removing nutrients from the system will develop. And the need for nitrogen and other nutrients by (algae is obtained from animal wastes, excreted into the water environment, a truly symbiotic relationship.

Dr Adey reasoned that by providing a surface that would facilitate algal growth, it could be possible to use fil amentous turf algae as a ‘filter’ to remove animal nutrients from a closed aquarium. At the same time the algae also takes up carbon dioxide from, and releases oxygen to, the seawater. This led to the development of the Algal Turf Scrubber – a tray in which a mesh screen is used as a substrate for algal growth.

While the basic principle was applied successfully to small1 model aquaria, its application to a large model was left to researchers at the’ Great Barrier Reef Aquarium, located in Townsville, Australia, a 4 million litre living coral reef aquarium and interpretive facility.

The Algal Turf Scrubber subsequently developed by the Townsville facility is a simple device that consists of a shal low tray with two removable coarse mesh screens and a tip bucket at one end, which uses the fulcrum principle for tipping water across the screen. Aquarium water is delivered to the tip bucket, which tips several times per minute, causing a series of waves to rush across the screens. This water turbulence provides good mixing to facilitate gas exchange and nutrient uptake. Algae in the process of growing on these screens, take up nutrients. Then, at regular intervals, the algae, with the incorporated nutrients, is scraped from the screens and discarded.

The use of 1000 micron window screen mesh, allows easy scraping of the algal filaments and provides a protective matrix in which the basal portions of the algae con grow. Once every 5 to 14 days the screen is removed from the tray, scraped with a piece of straight-edged perspex, rinsed and placed back in the tray. The algae that has been removed, along with the absorbed nutrients, is then dried in an oven and weighed, This is done to determine how efficient the scrubber is during various times of the year.

The algal turf scrubber is a way of increasing surface area for algal growth, to give it greater efficiency at removing nutrients. the Townsville facility uses 72 separate shallow trays (144sqm) situated on the roof of the building.

To simulate the natural processes of algae growth in nature, the Townsville facility found that they had to introduce water motion across the mesh tray to increase the rate of production and growth of algae.

Therefore, it is important to provide high water motion to increase the removal of nutrients from the water column. This is achieved using a tip bucket to deliver a pulse of water to the scrubber from the aquarium, creating a small turbulent wave, if you like, to evenly distribute the water across the filamentous algae. This is similar to wolves crashing on the crest areas of natural coral reefs. Outflows in the form of stand pipes, control the level of water in the tray and allow the water to return passively to the aquarium with out overflowing the scrubber.

Each scrubber has a settling bin incorporated into the out flows from the tray. The reduction in wave energy across the algal turf screen helps sediment settle out into the lower arects of the bin, and the water flows out through an elongated stand pipe or weir at the top of the settling bin. Sediments care then drained from the bottom of the settling bin through a valve.

Up to two pumps continually deliver water to the algal turf scrubbers, with all 1.4 million litres of water recycled through the scrubbers cat least once every 24 hours.

High Intensity 1000-watt metal halide lights are used to mimic the high light levels from the sun, and to compensate for cloudy days. They are also used to extend the effective day length for photosynthesis. The Townsville facility uses seventy-two 1000 watt metal halide lamps, at a distance of 0.5 metres above the algal turf screen, to maintain an effective photosynthetic rate. The lights are on for 6 hours per day.

Combined with daylight, this gives each screen 18 hours of illumination each day, increasing the growing period and the nutrient removal from the aquarium tanks.

To date, the patented Algal turf scrubbers have been used in association with six ecosystems – three coral reefs, one salt marsh estuary, one northern temperate littoral system, and one tropical mangrove. They have all been quite efficient at nutrient removal and can be easily manipulated to remove the desired quantity of nutrients in the water.

Perhaps the most publicised algal turf scrubbers ore those used in the enormous Biosphere 11 enclosure, situcited in Arizona, USA. These scrubbers helped purify the water as it woes recycled over and over again through seven ecosystems (desert, rainforest, salt marsh, mangrove, coral reef, deep water holding tank and intensified agricultural area).

The performance of the Great Barrier Reef aquarium algal turf system has exceeded original expectations. This simple system is capable of maintaining low nutrient sea water, removing toxic metals, maintaining seawater pH and oxygen levels, together with possibly providing some degree of ultraviolet sterilisation.

The Townsville Great Barrier Reef Aquarium represents the first application of algal scrubber technology to large volume aquarium systems. Aquaria using conventional water purification methods (e.g. bacterial and mechanical filters) generally have nutrient levels in parts per million (Spotte 1979), while algal scrubbers have maintained parts per billion (ppb) concentrations, despite heavy biological loading.

The success of the algal scrubbers in maintaining suitable water quality for the coral reef, is evident in observed spawnings of corals and many other tank inhabitants at the some time as they occur in nature.

The basic concept of using algae to remove nutrients as a filtration device, can be adapted to many applications. And it is applicable to both fresh and marine water.

The algal turf scrubber has come under close scrutiny since its incorporation into the Great Barrier Reef Aquarium. According to the Aquarium’s Research Officer, Paul Hough, such systems are capable of removing heavy metals, fertilisers and human wastes from waterways. However, finding a use for the harvested algae still remains unresolved, although large quantities of algae from these systems may become important in the production of energy through methane gas conversion and ‘gasahol’, as oil reserves around the world become depleted.

Ways of exploiting this natural process of algae cire being investigated across a wide range of industries, including agricultural. For the hydroponicist, it may provide the solution to removing nutrients from spent solutions, before recycling the water again through a closed system, using a fresh batch of nutrients.

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