Management of solids produced

3. Management of solids produced

3 . 1 . Feed quality In addition to treatment technology, waste management techniques can be employed to reduce the production of particles in the first place, or to facilitate their removal should their production be unavoidable. Improved feed quality in recent years has resulted in enhanced bio-availability of phosphorus and proteins, at lower concentrations. This results in a reduction in the quantity of faecal solids produced. Improved pellet integrity, with subsequent slower breakdown rates, further reduces feed losses. Optimised feeding systems and protocols have also reduced wastage. The close connection between feed quality and feed-derived waste production has been demonstrated in several reports, such as Cho et al. 1991 and A , sga˚rd and Hillestad 1998. The development of ‘high-energy diets’ with increased fat content, reduced carbohydrate levels, reduced protein levels, and improved digestibility has significantly decreased waste production in salmonid farming. In a standard diet for salmonids, the following fractions of the main components were shown to be indigestible and excreted as faecal waste: 13 of the protein, 8 of fat, 40 of carbohydrate fibre completely indigestible, 17 of organic matter, 50 of ashes and 23 of dry matter A , sga˚rd and Hillestad, 1998. About 40 of ingested protein N is excreted as dissolved N TAN = NH 3 + NH 4 + by salmon. Recent studies, indicate that a minimum of 11 g kg − 1 dietary P is required by juvenile Atlantic salmon A , sga˚rd and Shearer, 1997. The daily nutrition discharges per fish DND for nitrogen and phosphorus are predicted by the following equation Einen et al., 1995: DND N, P = nutrient fed − nutrient gain 1 where nutrient fed = ration fed g × nutrient in feed g g − 1 diet 2 and nutrient gain = growth g × nutrient in fish g g − 1 fish 3 At a feed conversion ratio FCR of 1.0 kg feed kg − 1 gain, the estimated discharges from juvenile salmonids, in terms of g N, P kg − 1 fish gain, are about 33 g N 26 g dissolved and 7 g solid-bound and 7.5 g P 80 – 90 solid-bound. Based on digestibility estimates of typical diets A , sga˚rd and Hillestad, 1998, the calculated discharge of suspended solids from salmon and trout farms should be 150 – 200 g SS kg − 1 fish gain at a FCR of 0.9 – 1.0. 3 . 2 . Feeding management Clearly the best means of reducing the quantity of waste discharged by an aquaculture facility is to reduce its production in the first instance. The feeding regime and technology used to both deliver rations to the stocks and monitor its intake can be used to minimise waste losses, as described by many authors specialising in feeding management, such as Alana¨ra¨ 1992, Durant et al. 1995, Summerfelt et al. 1995 and Derrow et al. 1998. Connections have been demon- strated between feeding management practices and either production economics and efficiency Storebakken and Austreng, 1987; Hankins et al., 1990, or waste production Seymour and Bergheim, 1991; A , sga˚rd et al., 1991; Westers, 1992. A full evaluation of different feeding regimes is outside the scope of this review. The concept is however, that feed utilisation should be maximised by supplying feed to the stock, such that the uneaten quantity is minimised. The required capacity of treatment systems can then be minimised, thus reducing capital and operating costs. Additionally, in water reuse systems that generally have a fixed carrying capacity based on the ability of the system to handle the daily feed application, removing the waste feed load can effectively increase fish rearing capacity Summerfelt et al., 1995. Technology for monitoring uneaten pellets has been shown to be a useful means of reducing wastage Summerfelt et al., 1995. Durant et al. 1995, Summerfelt et al. 1995 and Derrow et al. 1998 described devices that use ultrasound to detect feed particles. When pellets are detected in the tank effluent, the devices discontinue feeding. A pre-set timer is then activated to control the interval between feedings. Whilst these studies indicated an increase in growth of fish using these devices of up to 60, compared with ration feeding fixed portions, the effect on the solids loading in the effluents was not quantified. As well as optimising the timing of feeding, the location of feeding can affect both the quantity of solids wasted and their distribution within the culture facility. Many fish for example will not take feed pellets off the bottom, so tank hydrody- namics, pellet structure which affects the sinking rate and location of the feeder, need to be adjusted in order to maintain the solids in suspension as long as possible. Auto-feeders that broadcast pellets to the centre of a tank, or to the effluent end of a pond may be used to reduce the length of time the pellets remain within the culture facility, but also reduce the time available for the stock to eat those pellets. 3 . 3 . Flow management Culture stock density increases and water resource shortages have in some cases necessitated the use of oxygen addition and CO 2 removal systems. Wastewater flow rates from these systems tend to be markedly reduced, with a subsequent rise in waste concentrations. Higher solids concentrations are usually easier to treat in order to obtain significant benefits in wastewater quality. The specific water consumption in intensive farming has been reduced during the last decade by the use of supplemental oxygen. This affects the within-tank water quality and the self-cleaning efficiency of the tanks e.g. Colt et al., 1991; Fivelstad and Binde, 1994. The water flow can be reduced from 1 – 2 l kg − 1 min − 1 down to about 0.2 l kg − 1 min − 1 , or even lower, by using inlet water supersaturated to 160 – 200 oxygen saturation. Consequently, the tank outlet solids concentration is then a factor of 2 – 5 times greater than without oxygenation. Fivelstad and Binde 1994 found that a water flow below 0.16 – 0.20 l kg − 1 min − 1 reduced growth and caused tissue damage in freshwater fish. The concentration of carbon dioxide in freshwater should not exceed 10 – 20 mg l − 1 in freshwater Colt and Watten, 1988. A reduced water consumption, often by combining recirculation and addition of oxygen, is a means to improve the utilisation of the water supply and to reduce the discharged effluent load because of improved treatment efficiency Cripps and Bergheim, 1995. 3 . 4 . Variation in solids loading Several parameters have been found to influence the waste load, and hence cause variability in the quality of the wastewater. Kelly et al. 1994 found that the waste quantity discharged from a fish farm increased with temperature. Foy and Rosell 1991b however showed that the proportion of nutrients in the particulate fraction increased with temperature. These nutrients were not then becoming more soluble with increasing temperature as may have been expected. At sites where water temperature changes throughout the year, care must be taken at the planning stage of development, to ensure that the capacity of treatment facilities will be adequate to consistently produce the required discharge or recirculation water quality. Changes in ionic composition, dissolved nutrient concentration and physical parameters, such as flow rate and scouring, also influence the partitioning of nutrients between the dissolved and particulate phases, as reviewed by Hamann et al. 1990. Again changes in these parameters will form design criteria. Intermittent solids loading increases can occur as a result of intermittent tank cleaning operations Kelly et al., 1997, or from unit processes that function irregularly, such as back-pressure activated rotating micro-screens see below. Studies also indicate the advantage of continuous pre-treatment to concentrate wastes Twarowska et al., 1997. Bergheim et al. 1998 further provided evidence that if the waste solids concentration is increased, then the efficiency of the clarification processes will increase see below.

4. Pre treatment