Making good use of nutrients benefits the environment

When evaluating the environmental sustainability of aquaculture operations water consumption and water discharge quality are important criteria. Today water usage is governed by more or less strict rules almost everywhere in the world. For aquaculture, this often means the obligation to clean and treat the water discharge in such a way that natural ecosystems remain unaffected.

The combination of aquaculture (production of aquatic organisms) and hydroponics (plant growing without soil substrate) is called "aquaponics".

Water is essential to aquaculture: it is used for farming fish and other aquatic organisms and then returned to the cycle of nature. During the course of its use the water is contaminated with various substances that can reduce its quality and pose certain risks to ecosystems and to our drinking water. The effluent from fish farms is in no way comparable with effluents from industry or from car washes, however, because it mainly contains nutrients, mostly nitrogen and phosphorus compounds. That is to say exactly the same pollutants as are discharged into many rivers and lakes from agriculture during the fertilization of arable land and as a result of livestock. If too much nitrate and phosphate gets into the water this leads to over-fertilization and then eutrophication with excessive plant growth. This does not only damage the ecosystem but can also have a detrimental effect on biological diversity. Externally visible signs of such changes include increased silting of water bodies and mass development of micro-algae, known as algal blooms or "red tides" (this term comes from the brown or red coloration of the water during algal blooms, often caused by dinoflagellates), which are even sometimes toxic and can cause mass mortality of fish.

In contrast to agriculture, where nutrients mostly diffuse into neighbouring water bodies and are thus difficult to quantify and rarely assignable to specific polluters, the monitoring of the water discharge from aquaculture facilities is relatively simple. There can be considerable differences between individual aquaculture enterprises, however, depending on the type of fish produced, location, size, and intensity of production. Some facilities get their water from surface water, mainly lakes or rivers, others use groundwater that comes from a spring or is pumped up from the depths. Net cages, which are "open systems" that interact with the environment are to be evaluated differently from traditional farming in ponds that require little water to replace evaporation and seepage losses. Raceways need far more water than closed recirculating systems, whose water is treated after use and can be used several times. Maintaining healthy water quality in natural aquatic ecosystems is a primary goal of national and European legislation. That is why effluent quantities and nutrient loads are mostly subject to official regulations, compliance with which is checked regularly.

Discharges into the public water network presumably have to be approved in all developed countries. The National Pollutant Discharge Elimination System (NPDES) of the United States, for example, defines binding water parameters, criteria and standards that have to be adhered to by all dischargers, regardless of the technical difficulties and cost this might entail. Although the implementation of these requirements is the responsibility of the federal states, most farm operators adhere to the guidelines which include treatment specifications and limits for 126 of the most important pollutants. Although aquaculture facilities are not suspected of discharging harmful pollutants, they too need an NPDES permit to discharge their effluent into the environment. When applying for the permit it is necessary to specify in detail which substances are expected to be included in what concentrations in the water discharge. The granting of the discharge permit is subject to compliance with certain water values and treatment regulations. This is intended to reduce the overall discharge of suspended solids and non-conventional pollutants, to which nutrients such as nitrates and phosphates belong.

In Australia, the use, discharge, safeguarding and protection of water bodies are regulated in the Environment Protection (Water Quality) Policy EPA 488/03i. This policy defines clear quality criteria and maximum discharge quantities, and lists prohibited pollutants for marine areas, coastal areas and the groundwater and surface water bodies of the mainland. All dischargers are required to manage, reduce and control contamination of natural water bodies in order to meet the requirements on water quality. Contaminated water discharge should as far as possible be avoided or reduced so that the water is safe for reuse.

 

Settling tanks are an effective method for smaller aquaculture enterprises to reduce the presence of particulate substances in the water discharge.

 

Pollution of water discharge mainly consists of nutrients

Water discharge from aquaculture facilities mainly contains nitrogen and phosphorus-based nutrients that come from fish excrement and uneaten feed. These occur both in dissolved and solid (particulate) form. In addition, however, it may still contain other substances and impurities – although usually in very small amounts, for example, parasites, bacteria, fungal spores and other microbes, or possibly chemicals that are sometimes used at the farms. These include detergents and disinfectants for removing algae and microbes from equipment, nets and tanks, or narcotics with which the fish are sedated during transport or handling. And pigments, vitamins and minerals from the feed that dissolve in the water can also sometimes be detected, or medicaments, including antibiotics if they were used for treatment of diseases in the fish.

Which of these contaminants and how much of them enters the water depends on several factors, for example on the composition of the feed, the species and size of fish, but especially on the type and intensity of production and the biomass of the fish population. The larger the biomass of fish per cubic metre, the more feed is administered daily, the more the fish excrete and the higher is usually the level of water pollution. Despite high fish densities, however, the concentration of impurities in the water can be relatively low. If a farming facility operates a raceway with a high density of fish and large amounts of water flowing through it there will often be less impurities in the water than in systems that produce a relatively low quantity of fish but with less water. When estimating an aquaculture operation it is thus necessary to view the total load of pollution that it causes in natural waters. To compensate for possible environmental damages many states impose wastewater charges that are generally based on the substances that are released into the environment and the damage classes to which these belong. It is hoped that the resulting cost pressure will motivate the polluters to seek effective and inexpensive water treatment techniques.

Aquaculture discharges the same nutrients into the environment as agriculture and livestock farming

The indicated correlations between fish density and feed quantity, water throughput rate, degree and concentration of water pollution already indicate how difficult it can be in each concrete case to find and implement purification processes that are both efficient and cost-effective. In practice, a distinction is often made between systems that are "self-cleaning" such as raceways or circular tanks, and systems that are not. This latter group includes, for example, ponds. However, these terms can be misleading, because the "self-cleaning" capacity of a raceway is limited to the rapid and continuous discharge of dissolved and particulate nutrients which prevents their sedimentation in the raceway and thus the threat of oxygen depletion. This does not, however, mean that the pollutants have disappeared from the water or that the water had as it were "cleaned itself." In contrast, in “non-self-cleaning” systems particles of excrement and uneaten feed settle on the bottom as sludge and are not removed until the ponds are cleaned. Until then, however, a considerable part of the sedimented particulate nutrients dissolves again and can cause algal blooms.

 

Technical options for reducing water pollution

The efficiency of many water purification processes depends largely on the amount of time that elapses between the moment the contaminants (e.g., uneaten feed, the fishes’ excrement, excretion of ammonium through the gills) are released into the water and their transport to a wastewater treatment facility. As long as the nutrients and other contaminants are still present in particle-bound form they can be relatively easily and completely removed from the water. An elegant and relatively inexpensive method for concentration, thickening and elimination of nutrients is to use settling tanks that can be built in different ways. The basic idea behind these purification systems is to pass polluted water from the fish tanks into a separate tank where it will remain until the coarse and fine particles have settled to the bottom and can be removed. The effectiveness of the settling tank depends on the time the water spends within it and the time required for sedimentation, and ultimately on the relative size of the tanks compared to the total volume of water in the farming facility. The space needed to install such a system is not available everywhere, however. In pond farming the lowest pond is thus often used as a settling basin or even converted into a wetland to remove most of the dissolved nutrients from the water and convert them into plant biomass. Combined systems consisting of sedimentation and plant ponds can significantly improve nutrient retention. Another method for eliminating the dissolved nutrients is to use the water discharge for irrigating agricultural land. This does not only water the crops but also fertilizes them.

The IMTA salmon farm of Cooke Aquaculture combines salmon farming (rear) with the production of mussels (centre) and algae (front).

For flow-through systems such as raceways with high water throughput settling tanks are only of limited use, however. The water quantity flowing out of the raceway is comparatively high and the concentration of nutrients contained in it low. A settling tank would have to be exceptionally large to guarantee the length of time necessary for the sedimentation of particulate matter and thus a measurable cleaning effect. However, because this space is rarely available such farming facilities usually make use of separating techniques such as decanters, gravel filters, centrifugal and plate separators, protein skimmers, flotation or chemical flocculation. The most important purification technology, however, is probably microscreens which are mostly used in the form of drum filters, as well as disk or belt filters. Microscreens require a fair amount of maintenance and energy but are characterized by reliably good and consistent cleaning results for particle-bound nutrients. These "coarse contaminants" are also removed from the water discharge very quickly, which counteracts re-dissolution of nutrients, a process which is strongly time-dependent. The mechanical cleaning effect mainly depends on the mesh size of the filter screen, which is in practice mostly between 60 and 100 microns. This enables up to 90 percent of particulate matter and 80 per cent of biochemical oxygen demand (BOD) to be removed from the water. By using extremely tight mesh sizes of 40 microns it is even possible to reduce the risk of infection due to some parasites such as Ichthyophthirius multifilis (which can measure up to 1 millimetre) and a certain proportion of Saprolegnia infections, for example (in combination with UV light or ozone).

Although microscreens have no direct influence on dissolved nutrients they do, however, reduce these loads indirectly through the rapid mechanical removal of fish excrement and uneaten feed, thereby relieving the strain on biological filters, especially in recirculating systems (RAS) with internal water recycling. To prevent clogging of the narrow meshes by the separated solids, the fine mesh material which is made of plastic or stainless steel is at intervals automatically cleaned by a backwash rinse system. Depending on the fish population and amount of feed the water required for this is on average between about 1 and 2 per cent of the amount of outflowing water that passes through the microscreen filter. During this process the separated particulate dirt, i.e. the solid filtrate, is partly re-suspended. It thus seems reasonable to thicken the relatively liquid slurry to increase the dry matter content and reduce the volume of sludge. This secondary sedimentation can take place, for example, in a second downstream filter screen or in a funnel-shaped settling tank, which is called a "Dortmund tank" after the location of its first use in Dortmund in Germany 1887. In a Dortmund tank the slurry water is fed from below into the tank, whereby the flow rate decreases on the way up because of the widening of the funnel. This calms the water, the solids settle out and sink down the inclined side walls to the bottom where they can be removed.

 

The nutrient-rich sludge that accumulates at the bottom of the settling tank can be used in agriculture as a valuable fertilizer.

Making specific use of the economic potential of water discharge

Like unprocessed water discharge, sludge from aquaculture facilities still contains significant amounts of nutrients which lead to eutrophication of natural waters if their release is not controlled. It can also, however, be used profitably for the production of secondary products. If thickened, the sludge can for example be used directly as a substrate in biogas plants ("fish manure"). This also applies to plant biomass that grows in tertiary treatment ponds or plant settling ponds thanks to the nutrients dissolved in the water discharge. The production of crops that can be marketed directly can be even more lucrative. A nice example of this is the culture of aquatic and marsh plants on coco mats that grow in the water discharge from a zander RAS in southern Germany. Once the plants have grown and taken root the mats can be picked up and rolled out like lawns and used for initial plant growth on re-naturalized river and lake banks.

Combining fish farms with mussel and algae cultures in IMTA (Integrated Multi-Trophic Aquaculture) is also an attractive option. The Canadian salmon producer Cooke Aquaculture operates such a facility off the coast of New Brunswick. At the centre of this pilot project, which is one of the world's first IMTA salmon farms, is a salmon farm around which several mussel farms are located. These filter the organic excrement and feed residues that occur during salmon farming out of the water and use them for their own growth. In a second ring around the salmon farm and mussel cultures, algae cultures have been set up: their growth is partly supported by the inorganic excrement of the salmon and mussels which contain nitrogen and phosphorus. The IMTA principle thus combines the rearing of salmon, whose feeding leads to a lot of nutrients entering the ecosystem, with the production of "extractive" species such as mussels and algae that convert at least a certain share of this nutrient input back into biomass. IMTA systems thus contribute towards improving the quality of the water and the environment and also open up the opportunity to expand the product offering to include additional species.

Similar approaches were pursued by the international project ZAFIRA (Zero discharge aquaculture by farming in integrated recirculating systems in Asia) from 2002 to 2006, which attempted to combine modern Western RAS technologies with traditional Chinese methods of integrated agriculture based on minimum discharge of nutrients. Basically, in such projects it is usually a matter of using the "waste" (or nutrients) from one farming process as the basis for the next production step. In the ZAFIRA project the sludge from fish farms was used for bacterial production of single-cell proteins as well as sea cucumbers.

In Europe, aquaculture researchers’ fascination with closed material and energy cycles is to be seen in numerous aquaponics projects that are currently experiencing a tremendous boom. A project which is costing almost six million euros is probably of particular significance here. 18 partners from ten countries are taking part in the EU project INAPRO ("Innovative model and demonstration based water management for resource efficiency in integrated multitrophic agriculture and aquaculture systems") which is scheduled to take four years. In four demonstration projects, the technical and economic feasibility of aquaponics systems is to be proved and some of the obstacles that face this technology in practice removed. As we know, the step from the "research playground" to commercial breakthrough is otherwise often too big for smaller companies.