ISSN 0704-3716 Canadian Translation of Fisheries and Aquatic Sciences (I-1 No. 5252 Reduction of the loading effects of fish farming -- a literature review E. Parjala Original title: Kuormituksen vahentaminen kalanviljelyssa, kirjallisuuskatsaus In: Vesitalous 5: 11-16, 1984 Original language: Finnish I Fieseei'.lieei-,'s gt (e.nnans 04 6.i.-7ReeiY 1 OM iu imfg ifileipeimilinitee gz ce.a.eil -ns,,...,,,,..:.........,73:'e.l...trer...s.,,, Available from: Canada Institute for Scientific and Technical Information National Research Council Ottawa, Ontario, Canada KlA 0S2 1986 17 typescript pages
14, Secretary Secrétariat of State d'état MULTILINGUAL SERVICES DIVISION DIVISION DES SERVICES MULTILINGUES TRANSLATION BUREAU BUREAU DES TRADUCTIONS.LIBRARY IDENTIFICATION - FICHE SIGNALÉTIQUE Translated from - Traduction de Finnish Author - Auteur Erkki PARJALA Into - En English Title in English or French - Titre anglais ou français Reduction of the loading effects of fish farming - a literature review Title in foreign language (Transliterate foreign characters) Titre en langue étrangère (Transcrire en caractères romains) Kuormituksen vahentaminen kalanviljelyssa, kirjallisuuskatsaus Reference in foreign language (Name of book or publication) in full, transliterate foreign characters. Référence en langue étrangère (Nom du livre ou publication), au complet, transcrire en caractères romains. Vesitalous Reference in English or French - Référence en anglais ou français Water Economy Publisher - Editeur Place of Publication Lieu de publication Finland Year Année 1984 DATE OF PUBLICATION DATE DE PUBLICATION Volume Issue No. Numéro 5 Page Numbers in original Numéros des pages dans l'original 11-16 Number of typed pages Nombre de pages dactylographiées 17 Requesting Department Ministère-Client DFO Translation Bureau No. Notre dossier no 2909915 Branch or Division Direction ou Division IPB Translation (Initials) Traducteur (Initiales) LT Person requesting Demandé par J. Morrison Your Number Votre dossier no Date of Request Date de la demande 16.09.1986 SEC 5-111 (81/01)
Sec' retary Secrétariat 149 of State d'état MULTILINGUAL SERVICES DIVISION DIVISION DES SERVICES MULTILINGUES TRANSLATION BUREAU BUREAU DES TRADUCTIONS Client's No. N du client Department Ministère Division/Branch Division/Direction City Ville, DFO IPB Bureau No. No du bureau Language Langue Translator (Initials) Traducteur (Initiales) 2909915 Finnish LT Vesitalous 1984 Vol. 25 (5) pp. 11 16 Parjala, E.: REDUCTION OF THE LOADING EFFECTS OF FISH FARMING A LITERATURE REVIEW? Canad'a SEC 5-25 (Rev. 82/11
1. GENERAL Much attention has been paid lately to the loading effect of fish farms on waters. There is a general trend towards reducing the load, especially that caused by phosphorus. Therefore, testing and development of different wastewater treatment methods have been carried out at fish farms. Initiatives have been made to find more comprehensive means of reducing the load by considering the entire establishment and its operation. This has resulted i.a. in the development of new types of farms, rearing structures, and feeds (Kera 1983). The creation of efficient wastewater treatment methods has been limited by the very large quantities of water involved (the biggest farms use several m 3 per second) and the very low contents of contaminants in the wastewater (clearly lower than e.g. in municipal wastewaters) (Parjala et al. 1984 b). The present article evaluates, primarily on the basis of the literature, the applicability of the different methods for reducing the loading effect of fish farming. 2. SEDIMENTATION In view of cost/benefit considerations, sedimentation has been regarded as the most advantageous and realistic method for treating wastewaters from fish farms (Warrer Hansen 1982). However, in establishments with earthen ponds and channels, sedimentation leads to the problem that the rearing ponds themselves act as settling basins on the bottom of which the sludge accumulates. It is true that the sludge gradually becomes suspended in the water again, but when the suspended solids reach the settling basin at the end of the installation, they are as a rule SQ poorly settleable, that only a very small portion of the solids are retained in the settling basin. The following is an example of solids removal from a traditional earth pond and a circular self cleaning rearing tank (Warrer Hansen 1982) :
Earthen pond Circular self-cleaning tank 3 3 min 6 min 11 min 15 min 30 min 14% 21% 30% 597e 29% 37% 63% 66% 37 % 73 % The following example illustrates also the efficiency of solid matter separation in earthern pond and circular self-cleaning tank, for different feed types (Warrer-Hansen 1982) : Earthen pond + settling basin Circular tank + settling basin Retention time in settling basin 15 min 30 min 5 min 15 min 30 min fresh feed half-dry feed, 12% 15% 19% 31% 51% 57% dry feed 37% 37% 59% 66% 67% 72% Thus, a circular rearing tankis obviously superior to an earthen pond in view of solid matter removal. However, according to the present consensus, large fish (:>100 g) can not be reared in circular tanks successfully (Jaakkola 1981). If self-cleaning rearing tanks can be used (as fry farms use them right now), a settling basin alone will give good treatment results with regard to the removal of solid matter and BOD and fairly good results with 7 regard to the removal of nutrients as well (Liao 1970). The structure and the design of the settling basin have been discussed i.a. by Warrer-Hansen (1982). In view of the theory of surface load, the design flow rate has to be reduced by a factor of 1.5 to eliminate turbulence. The theory of surface load implies that the depth of the basin would not be important for the treatment efficiency of the basin. On the other hand, the flow in the basin should not exceed 2-4 cm/s, which may set certain limits for the basin depth and retention time as well. A horizontal settling basin is expected to fulfil the following requirements : Width / length = 1/3-1/7, depth = 1.5-2.5 m, surface loadc1.5-1.0 m/h, retention time 1.0-1.5 h, and Re-number tej.:. 10 000. The settling velocity of a particle just retained in a basin is roughly obtained from the ratio of flow rate Q (m 3 /s) to basin surface A (m 2 ), that is, Q/A.
Figures 1 and 2 illustrate the significance of solid matter removal for reduction of the other load parameters and the relation between removal percentage and retention time of solids in different types of rearing enclosures. According to Muir (1982), the required total volume V t (L) of a settling basin is obtained by means of quantity of fish S (kg) at the farm, fish density in relation to flow rate P f (kg of fish /I, H 2 0 / min), basin retention time& (min), and inlet and outlet zones of the basin : volumecoe ff icientsk.and k o of the 1 V t = (Se/ P f ) (L + k. + k). o The required basin retention time can be estimated on the basis of the settling velocity of solids. The volume coefficients can be estimated on the ground that in general, 1 5 % additional space is needed for basin inlet and outlet zones to regulate the flow. If a farm's water is aerated, the ratio of fish density to flow rate can be increased and the required basin volume decreased. For example, if P is doubled, V is reduced by half. On the other hand, part of the solids f t may become diffused, which means that basin retention time e must be corre spondingly lengthened. If lamellate basins are used for settling, which means that the settling area grows by factor F a, the following is obtained : V t =-(Se/Pj a )(LA- 1(.+1( o ) 1 E.g., if F a = 10 (10 lamellae), the area required is reduced to 1/10. If solids can be collected in the main flow to a certain spot, from where they can be removed by means of a secondary flow, the situation can be described by means of concentration factor F. If it is assumed that the secondary flow catches all solids from the wastewater of the farm, the following expression can be used : v t = (seef c /P f )(L+ k. + k ) o
If the secondary flow is 10 % of the main flow, F c = 0.1. If the secondary flow does not catch all solids, the basin retention time must be increased and the treatment expanded. It is to be noted that the first stage of solids removal does not much depend on the solids content. However, the removal of soluble substances depends on the content in accordance with the order of reaction, which means that the area, volume, etc. required by the treatment increases with decreasing content. It is questionable whether settling basins should be built in fish farms with long retention times (several hours). The efficiency of the treament would typically be: solids 10-15 %, total phosphorus 0-20 %, total nitrogen 0-30 %, phosphate-p 0-20 %, and BOD 0-25 % (i.a. Kekkonen 1982, Selanne et al. 1983, Parjala and Tamminen 1984). Fish. farms use a large variety of settling and sedimentation basins. The simplest of them are mere earth-bottom ponds. The hydroclone is a recent application characterized by a small area (about 1/10 of the area required by traditional sedimentation basins) and large surface load. However, the hydroclone is suited only for rapidly settling solids (velocity of settling over 0.5-0.6 cm/s)(parjala and Tamminen 1984). The traditional horizontal and vertical settling basins would obviously remove more efficiently light-weight solids with a small particle size, but they are expensive and require a large area (Mashauri 1981). 3. FILTRATION 3.1 Peat filtration The filter capacity of peat is based on : - mechanical filtration - biological filtration - surface activity (adsorption) - cation exchange
- water-retention capacity - evaporation. The dimensions of a peat filter are decided primarily by the filter structure, the capacity of the peat to retain contaminants, and the water permeability of the peat. Below are some general instructions concerning peat filter construction (Selanne et al. 1983) : 1) peat layer thickness must be 30-50 cm 2) peat layer must lay on drainage gravel and underdrains 3) sludge water must be conducted to the filter evenly by means of a distributor disc or a feeding plate 4) peat must be compacted by wetting it uniformly so that the filter develops no shortcuts and that it does not float 5) basin has to have earth walls so that when the peat dries, no cracks will form at the edge to allow shortcuts 6) filter has to have sufficiently high edges to prevent overflow 7) it must be possible to monitor the quality of the filter water 8) underdrains must have enough inclination to prevent clogging. According to Selanne et al.(1983), the preliminary design value can be set at 200 g total phosphorus / m 3 peat. The following treatment results have been obtained in different tests (Selanne 1982, Selanne et al. 1983) : solids 40-99 % BOD 7 62-91 % Tot. N 60-96 % Tot. P 52-96 % PO -P 4 39-97 % The retention capacity of peat filters is primarily determined by peat species, degree of peat humification, and the physical and chemical properties of the peat. The cation exchange capacity of Finnish peat is generally 50-150 me/100 g, while its water permeability is 10-8 - 10-2 cm/s (Seppanen et al. 1982). No large-scale practical experience with peat filtration exists at present. As it is, the method is applicable primarily to small farms only because
of the limited retention capacity and water permeability of peat. However, peat filtration is a suitable method for further processing and treating/ condensing sludge water pumped from sedimentation basins. 3.2 Mechanical fine filtration The USA has used, besides the hydroclone, also mechanical fine filtration for solids removal from wastewater. Its costs are slightly higher than those of the hydroclone, and the scope of its application is the same as that of the hydroclone as far as flow rate is concerned. As a rule, the mechanical fine filter, or micro-filter, consists of a cylinder covered with filter cloth or stainless steel screen with holes of a size of 20-120 )Am. Microfilters are either horizontally or vertically rotating. The cylinder is cleaned by backwash which can be continuous or intermittent. In field tests carried out by Drehwing et al. (1979), these filters obtained efficiencies of 32-58 %, while the cleaning efficiency of the hydroclone under similar conditions was 18-55 %. Norway has experimented preliminarily with different cylindrical filters (mesh size 0.20 mm) in fry farms, obtaining efficiencies of 40-60 % (Lygren et al. 1983). 3.3 Sand filtration Small farms may find it possible to use coarse sand filtration, even though the filter area would be large and the washing of the filter could be troublesome. Big farms and farms with recirculating water could install continuous sand filters, which need no backwash. These filters could be combined with cherdcal precipitation, without a need for separate basins for flocculation and sedimentation. 4. WASTEWATER LAGOONS If a fish farm has lots of space at its disposal, it can treat its wastewater by means of wastewater lagoons,where the water is treated biologically and mechanically (sedimentation). However, the laponswould have to be so large that the lagoon retention time, in view of the Finnish conditions (low water temperature) and the degree of dilution of the wastewater, would be several days (Liao 1979, Buhr and Miller 1983). Also, the lagoon should preferably
8 be no deeper than 40-50 cm. In practice, this system would require aerator-equipped channels with continuously circulating water. In an optimum case, the use of lagoons could yield a removal efficiency of over 60 % for BOD and solids, nutrients could be removed rather efficiently as well. The lagoons could function poorly in the winter, but on the other hand, the winter load would be small too. Wastewater lagoons combined with sand filtration could constitute a suitable solution for small farms. A 1-m,coarse sand filter could obtain removal rates of 0.1-0.5 kg BOD/m 2 /day and a lagoon correspondingly 0.02 kg BOD/m 2 /day (Muir 1982). Sand filter volume V f can be calculated from V f = SXK f /L f where L = load factor (e.g. 0.1-0.5) f S = fish mass X = production of BOD K = concentration factor (equivalent with reaction rate). f Correspondingly, volume V 1 required by the lagoon is obtained from V 1 = SXK 1 /L 1 where L 1 = load factor (e.g.0.02). In Finland, wastewater lagoons are associated with problems caused by the unfavorable temperature conditions, and also, when the wastewater to be treated is highly diluted, the retention times and lagoon areas required may become unrealistically large (Buhr and Miller 1983, Middlebrooks and Pano 1983, Uhlmann et al. 1983). 5. RECIRCULATION PLANTS A good way to solve the environmental problems caused by fish farming is to install a recirculation system, which practically eliminates the load on waters. A recirculation plant means, however, that the farm contains several water treatment processes that are precisely planned and regulated.
The treatment of the water recirculated in these plants includes as a rule removal of organic substances e.g. by sedimentation, flotation, or sludge activation methods, a biological filter for nitrification-denitrification of nitrogen compounds, a mechanical filter (e.g. fast sand filter, pressure filter, zeolite filter), sterilization of water by UV light, and possibly temperature regulation as well. The amount of fresh water needed for recirculation is usually a few % of the total amount of water. Recirculation plants are discussed in greater detail i.a. by Hagebro et al. (1983), Hodal (1983) and Jernelov et al. (1980). 6. COAGULATION In principle, the solids and the phosphorus contained in the wastewater can be removed by coagulation as well. However, the use of coagulation is limited because of the large water quantities and the low phos - phorus and solids content. The cost of the coagulationclemicals would be relatively high. Also, the coagulation basin must be relatively large (unless supplementary continuous sand filters are used), because the period of interaction between the coagulation chemical and the matter to be coagulated must be long, due to the low content. It is possible to use coagulation in a supplementary manner to increase the settling velocity of solids in sedimentation units and also to concetrate sludge water. This kind of application has been tried, but no large-scale experiments have been carried out, and experience is lacking. The coagulation chemicals to be used would be primarily FeSO 4, FeC1 3, A1(S) ) Ca(OH) CaO and polyelectrolytes. Favorable results have been 4 3' 2' obtained i.a. in preliminary trials performed at the Laukaa Central Fish Farming Plant (Selanne 1982, Selanne et al. 1983). However, no trials have been carried out on a practical scale and under practical conditions. The possible uses of coagulation at fish farms have been discussed also by Makinen and Naukkarinen (1982).
1 0 7. AERATION Aeration is another method for treating wastewaters. Aeration and oxygenation are already practised at fish farms to some degree, to improve oxygen conditions in rearing ponds and thus to increase growth. It has been proposed i.a. that aeration be carried out in a separate aeration basin at the end of a plant, where it:could be combined with coagulation of phosphorus (Anon. 1980). It would be possible to combine aeration with settling as well. However, aeration carried out prior to sedimentation may, by breaking up the frail organic solid particles, poorly-settling solids more difficult. make the separation of Aeration affects primarily the 0 2' CO 2, NO 2 and NH 3 contents. By aeration, the 0 2 content can be made to rise to its saturation value and the CO 2 content can be made to decline. Effective aeration makes NH 3 the most important nitrogen metabolism product of fish, oxidize to NO 3' which to some degree decreases the secondary oxygen consumption. Especially in earthen ponds and earthen channels, efficient aeration can promote the dissolution of NO 2' NO 3 and NH 4 from sludge and solids (Holleman and Boyd 1980). Research on aeration is still needed with regard to its economy, efficiency, and applicability in wastewater treatment. In any case, to be efficient, the retention time required by separate aeration would be rather long (several hours). Heerfordt and Hodal (1983) studied the efficiencies of different aerator types under standard conditions, primarily in view of increasing fish pro - duction. The principles of aeration and different types of aerators have been discussed in greater detail by Hiidenheimo et al. (1983). 8. POSSIBILITIES OF REDUCING PHOSPHORUS LOAD A reduction in the phosphorus content of feed from 1.5 % to 1.0 % implies a 45 % reduction in the phosphorus load to the water, provided that the other variables do not change. Correspondingly, a reduction of the feed coefficient from 1.5 to 1.0 decreases the phosphorus load to the water, calculated per kg of feed consumed, by 10-20 % and the phosphorus load per kg of fish produced by about 45 % (Makinen 1983).
11 Table 1 (Makinen 1983) shows an example of the importance of the phosphorus content of feed and of the feed coefficient for reducing the phosphorus load. If it is estimated that half of the total phosphorus load is in the solids to be settled, the total phosphorus removal efficiency depends decisively on the solids separation efficiency. Even though all solids could be separated, which requires that the sludge is fresh (less than a week old), half of the total phosphorus would still remain. Under the most favorable conditions, 20-25 % of phosphorus load can be removed from old, decayed sludge. Table 1 Example : a farm producing 100 t per year aosphorus content of fish l'hosphorus content of feed Feed coefficient Recovery of phosphorus as sludge Annual phosphorus load to watercourse 1.4% 0.4% 1.5% 1.0% 2.0 1.5 0% 50% 2 600 kg 550 kg Tâble 2 Phosphorus load per kg of feed, efficiencies at different sludge removal Source Feed Phosphorus load g/kg of feed Efficiency of sludge removal 0% 50% 100% Selanne et al. T40 4.7 3.0 1.2 (1983) T40 5.9 4.8 3.7 Tess 5.1 3.8 2.4 Forelli 6.7 5.4 4.1 Myllyla Selin et al. Sumari et al. Test feed Control feed Test feed Control feed 10.8 5.7 9.7 10.1 8.1 8.3 4.1 7.1 5.8 2.9 4.4 7.0 3.9 6.3 4.4 By a complete sludge removal, 30-80 % of phosphorus load could be removed (Makinen 1983).
80t 3..5 -et 60 240 0 1.11 r-1. 2,20., e: '... el.'.......,.../.,..-'...- _If., /".."-lee"., N11 3 -N (Liao & nlyo 1974)... œ ',/ 40D (niir 1978).'.'. 0015(muir & Lipper 1970) 50 100 kiintoaineen poistotehokkuus (%) solicit 'mote/vat eeicienc.ti NO3 -N, PO4-1' (nlir 1978) Fig. 1 Effect of solids removal efficiency on other load parameters (Muir 1982) E vi ipre (min ) 40 30 20 10 ean4 ponds maa-altaat (Warrer-Hansen 1979).///e / =6,19 4ainks kasvatussâilibt (»air 1978) 1f ' (rglaa r marine ponds rc;yiireiit kasvatusaltaat (Warrer-Hansen,1979) 50 100 kiintoaineen poistotehokkuus (%) çoedç reamoval e4ic-ievtui % Fig. 2 Effect of retention time on solids removal efficiency (Muir 1982)
41lEct wasktri weer suodatinten pesuvesi rermin eastin le, rn to...pt.,. «reef: UV-àuodatin aéltelex. lentnitn saatbykstkkô painesuodatin recur-et 411 rei- 2air 110.4 poistuva vesi was«utaar c:=> suodatin pyôrreselkeytin teekrocione laskeutus Fig. 3 Example of recirculation plant Table 3 Relative capital costs of different basins (Muir 1982) : Pond without embankment Embankment pond Brick, concrete, or metal pond or channel Fibre glass tank Filtration ponds 1.0 1.3 1.2 1.9 1.3 1.6
12 If sludge removal is carried out to remove phosphorus, a decision has to be made with respect to the method to be used for wastewater treatment. Letting sludgewater be absorbed in soil is difficult, because an area of 1 m 2 would be required for 1.5-2.0 m 3 of sludgewater (solids content about 7 'X). Peat filters could be used at small farms. In peat filtration, the maximum. total phosphorus load per 1 m 3 peat is 200 g. It is probably most practical to pump the produced sludgewater into tanks to which chemical coagulants are added (e.g. 2-4 kg lime/m 3 ). The sludge is then allowed to settle, after which it is transferred to a sludge bed to dry (Makinen 1983). 9. REMOVAL OF NITROGEN COMPOUNDS Zeolite filter (NA ((A10 ) (Si0 ) )), a method based on ion exchange, 12 2 12 2 12 can be used to remove nitrogen compounds and especially ammonia. It functions, independently of temperature, in the ph range 4-8, and it is not affected by other substances like biological filters are (Anon. 1983). The specific gravity of zeolite is 0.69 and its price about Fmk 35/kg (Hiidenheimo et al. 1983). Aeration (stripping) and nitrification can also be used to remove N113-N. Removal of nitrogen compounds from the wastewaters of fish farms is not, however, of as much current interest as is the removal of phosphorus, except in recirculation plants. 10. BIOMASS FOR NUTRIENT REMOVAL It has been proposed that the nutrients in wastewaters could be removed also by means of biomass, primarily by aquatic plants but also by means of the food chain plankton - benthic fauna - fish (Lindqvist et al. 1981, Puustinen and Lindqvist 1982, Hejkal et al. 1983). Removal of nutrients by aquatic plants requires a biomass basin with a depth not exceeding 1 m, which can at the same time function as a settling basin. According to Lindqvist et al. (1981), the biomass basin of a medium-sized fish farm should have a minimum area of 4 000-10 000 m 2.
13 When plans are made to remove nutrients by means of plants, the following points have to be considered : - plant species and their adaptability (plants with submerged or floating leaves tolerate harvesting best) - capability of plants to take up nutrients from water - production (kg/ha/year or kg/ha/day) and harvesting properties of plants - easiness of propagation in the beginning of growth period - water depth and flow velocity - water retention time and interaction time with plant mass - basin shape and degree of shadedness - nutrient content and physical quality of basin bottom The growth rate of the plants is affected primarily by nutrient ratios and quantities, plant density, amount of available solar radiation, and temperature. The critical N/P ratios and quantities that guarantee maximal production are not known for many plant species. For example, the optimum N/P takeup ratio is 3/1-5/1 for water hyacinth (Eichornia crassipes (Mart.) Solms) (Reddy and Sutton 1984). Plants are capable of using mainly only soluble nutrients in an inorganic form. Furthermore, many aquatic plants can remove nitrogen more efficiently than phosphorus (Reddy 1983). Thus, in many species the retention time needed for phosphorus removal is longer than that needed for nitrogen removal. Plantsabsorb primarily nitrogen in ammonia form but also nitratesand nitrites to some degree (Martin and Goff). Also, the optimum growing density of plants depends very much on the amount of available nutrients and the season. Maintenance of the optimum density throughout the growing period frequently requires a thinning of the vegetation during the time of the most rapid growth. Aquatic plants attain their maximum growth rate as a rule at a temperature of 25-30 C. When the temperature drops below 10 o C, the production of the plants decreases very rapidly to zero (Reddy and Sutton 1984). The low production at low temperatures is further emphasized, if the nutrient content of the water is poor. An efficient nutrient removal by plants requires a retention time in the biomass basin of several days (5-20 d) even under favorable conditions. The retention time required is longer during seasons with low temperatures.
14 The size of the biomass basin is determined primarily by the quantity of the water to be treated, the retention time required, and the depth of the basin. In FInland, the water quantitye be treated during a summer would be perhaps 100 200 m 3 /ha/d (Reddy and Sutton 1984). It is to be noted that at the end of the growing season, the biomass has to be removed from the basin, so that the nutrients stored are not released from the wilted plant matter and the basin bottom during fall and winter. It is obvious that the use of biomass for reducing nutrient loads is not an applicable method under the Finnish conditions, if used alone and in its present form, because of the low temperatures and the short growing season. The method is further impaired by the same factors as the conventional methods are affected by, namely low nutrient contents of water and large water quantities. However, little information or experience is available about the use of biomass for wastewater treatment in Finland, and therefore, it may very well be possible that the method can be adapted to use in Finland as well. The use of a biomass basin has been studied preliminarily at a Finnish fish farm (Puustinen and Lindqvist 1982). Experiences of summer 1981 proved that in the most favorable case, a maximum of about 10 % of total phosphorus load and about 5 % of total nitrogen load were removed by aquatic vegetation (i.a. choke pondweed) in the settling basin (about 0.8 ha, estimated flow rate 1.0 1.5 m 3 /s). However, the basin could retain nutrients only during mid summer, while in late summer the effluent contained more bound phosphorus than the influent. 11. COSTS Very little information exists on the costs of wastewater treatment at fish farms. These costs consist of the costs of water treatment and the costs of the basins, structures, and equipment required. The treatment costs depend decisively on the treatment unit and the treatment method. The basin costs are determined by the basin size and material. The capacity of the basin is also influenced by fish density per flow rate (kg/l/min) and volume (kg/m 3 ).
15 Muir (1982) estimated that at big fish farms, a 50 % increase in capital costs would achieve a solids removal rate of 30-40 % and a BOD removal rate of 20 % by single settling. In small farms, it would be possible to remove, by a 10-20 % capital cost increase, 10-20 % of solids and 5-15 % of BOD. Different water protection methods and their annual costs have been discussed by Makinen (1983) as well. 12. CONCLUSIONS On the whole, the loading effect of fish farms can be assumed to have decreased considerably e.g. from the 1970's, due to factors such as growing attention by the water authorities. It has been possible to reduce the load especially by indirect measures such as development of feeds and the feeding and farming technology. Also, water treatment and sludge collection methods have been developed. However, it is easier to essentially reduce the load in rearing ponds than at the end of a facility,by separate unit operations based on load quality and wastewater properties. As the traditional earthen pond and channel farms will probably continue long to be the dominant farm types in Finland, efforts should be made particularly to develop sludge separation methods for these farm types. At the moment, preliminary work is carried out to develop different types of sludge collection pockets etc. However, not enough attention has been paid yet to the hydraulics of the rearing pond and the behavior of solids and sludge on different interfaces. Studies of sludge behavior and basin currents would help to develop simple, easily applicable solutions for collecting the sludge to certain locations in a basin, from where the sludge could be removed in a relatively easy and inexpensive manner. Also, it has to be remembered that the treatment method to be chosen and its economy depend critically on the treatment efficiency required. Up to now, fish farming plants have been presented no demands mentioning figures concerning treatment efficiencies.
16 LITERATURE I) Alonen, K. 1981. Peat in wastewater treatment at fish farms. - Suomen Kalankasvattaja (Finland's Fish Farmer) No. 4:28-29. 2) Anon. 1980. A Finnish aerator doubles the production of fish farm and reduces environmental disadvantages. - Suomen Kalankasvattaja No. 2 : 16-17. 9 Anon. 1983. Elimination of ammonia by zeolite. - Suomen Kalankasvattaja No. 1 : 29. Hagebro et al. 1983. Fish farming in recirculation systems. - Progress 11) report by the Management Group of the Council of Technology for recircula- CDehil) tion in fish farming, 26 pp. Heerdefordt, L. and Hodal, J. 1983. Aeration systems for acquacultural 1) purposes, I. Testing of aerators under standard conditions. - Water (Dak.44) Quality Institute's report to the Council of Technology, 15 pp. Hiidenheimo, L., Mustajarvi, V. and Reiter, P. 1983. Increasing the oxygen content of water. - Suomen Kalankasvattaja No. 1 : 38-43. - Paper read during the Water Research Days 13.10.1983 in Kuopio, 5 pp. S) cl) lo) Jaakkola, M. 1981. Water treatment at fish farms : Solids removal. Suomen Kalankasvattaja No. 18-11. Kehitysaluerahasto Oy (Fund for Development Areas Co.), 1983. About the present state of Finland's acquaculture and proposals for its development. 40 pp. + 11 appendices, Kuopio. Kekkonen, I. 1982. A study of removal of sludge and its further processing on a practical scale. - Monitoring Seminar of Fish Farms, Saarijarvi 24-25.11.1982; Stencil, 3 pp. + 4 color appendices, The National Water Board. n) Lindqvist, 0.V., Puustinen, M. and Karenlampi, L. 1981. About the management of environmental issues in connection with fish farms. - Suomen Kalankasvattaja No. 1 : 11-12 and 18. ii ID) 10?) 15) - Building Technology Division of the Tampere Technical University, Water Technology Publication No. 7, 143 pp., Tampere. Makinen, T. 1983. Water protection in fish farming. - Suomen Kalankasvattaja No. 1 : 22-27. Makinen, T. and Naukkarinen, M. 1982. A model study on the applicability of hydroclone to wastewater treatment at fish farms. - Stencil Series of the National Water Board 128, 41 pp. + 5 app., Helsinki. Parjala, E., and Tamminen, A. 1984. A field study of the applicability of hydroclone to wastewater treatment at fish farms. - Kuopio University, 34 pp. + 4 app.
17 Parjala, E., Tamminen, A. and Lindqvist, 0.V. 1984. The loading effect /) of fish farms in inland waters, with an earth channel farm producing 200 t of rainbow trout annually as an example. Kuopio University, 77 pp. 19 Puustinen, M. and Lindqvist, 0.V. 1982. Nutrient discharges from fish farms and their reduction : Biomass to bind nutrients. Kuopio College, Institute of Applied Zoology, 84 pp. + 9 app. Selanne, A. 1982. Studies of phosphorus balance and peat filtration at 119 LKKVL* in summer 1982. Monitoring Seminar of Fish Farms, Saarijarvi 24 25.11.1982, Stencil, 20 pp. + 9 app., The National Water Board. 1&) Selanne, A., Makinen, T. and Helkio, R. 1983. Fish farming sludge and its further processing. Stencil Series of the National Water Board, 173, 105 pp. Seppanen, A., Assmuth, T., Jaaskelainen, K., and Cajander, V. R. 1982. 17) Environmental effects of peatland dumps, preliminary study. Ministry of Internal Affairs Publication A:18. Central * LKKVL possibly refers to the Laukaa risn Farms (Translator's note)
ERKKI PÂRJÂLÀ Kuormituksen vâhentâminen kalanviljelyssâ, kirjallisuuskatsaus 1. YLEISTÂ Viime aikoina kalanviljelylaitosten vesistôkuormitukseen on kiinnitetty paljon huomiota. Yleisenâ suuntauksena on ollut, ettâ kalanviljelylaitosten kuormitusta, erityisesti fosforikuormitusta, tulee pienentââ. Tama on johtanut erilaisten poistovesien puhdistusmenetelmien kokeilemiseen ja kehittâmiseen kalanviljelylaitosten yhteydessâ. Vâhitellen on mes alettu etsiâ kokonaisvaltaisempia kuormituksen vahentamiskeinoja huomioimalla koko laitos ja sen toiminnot. Tama on merkinnyt uusien mm. laitos- ja kasvatusallastyyppien sekâ rehujen kehittâmistâ (Kara 1983). Poistovesien tehokkaiden puhdistusmenetelmien lôytamistà on kalanviljelylaitosten yhteydessâ rajoittanut se, ettà laitosten kayttâmat vesirnâârât ovat hyvin suuria (suurimmilla laitoksilla useita kuutiometrejà sekunnissa) ja epâpuhtauspitoisuudet poistovesissâ ovat hyvin alhaisia (selvasti alhaisempia kuin esim. puhdistetussa yhdyskuntajâtevedessà) (Pârjalà ym. 1984 b). Tâssa artikkelissa arvioidaan lahinnâ kirjallisuuden pohjalta erilaisten menetelmien soveltuvuutta kalanviljelyn kuormituksen vahentamiseen. 2. SELKEYTYS Selkeytysta on pidetty kustannus-hety -tarkastelun pohjalta edullisimpana ja realistisimpana tapana puhdistaa kalanviljelylaitosten poistovesiâ (Warrer- Hansen 1982). Selkeytyksen ongelmana maa-allasja -uomalaitoksilla on kuitenkin se, etakasvatusaltaat itse toimivat laskeutusaltaina, joiden pohjalle syntyva liete kertyy. Vâhitellen liete tosin alkaa suspensoitua uudelleen veteen, mutta kun suspensoitunut kiintoaine saavuttaa ERKKI PÂRJÂLA, FM Te- ja teollisuushygienian laitos Kuopion Yliopisto laitoksen loppupââssa olevan selkeytysaltaan, se yleensâ on jo niin huonosti laskeutuvaa, ettâ selkeytysaltaaseen pidattyy vain hyvin vahainen maârà kokonaiskiintoaineesta. Seuraavassa on esimerkki syntyvan kiintoaineen poistumisesta perinteisestâ maa-altaasta ja pereasta itsepuhdistuvasta kasvatusaltaasta (Warrer- Hansen 1982): maa-allas pereâ kasvatusallas maa-allas + laskeutusallas pyareâ allas + laskeutusallas 3 min 14 % 30% Kiintoaineen erottumisen tehokkuutta maa-altaan ja perean itsepuhdistuvan kasvatusaltaan yhteydessâ laskeu- viipymâ laskeutusaltaassa 15 min 30 min 5 min 15 min 30 min Siis pyôreâ kasvatusallas on selvâsti maa-allasta parempi ratkaisu kiintoaineen poiston kannalta. Taman hetkisen kasityksen mukaan kuitenkin suurten kalojen ( > 100 g) kasvattaminen pereissâ altaissa ei onnistu (Jaakkola 1981). Jos itsepuhdistuvia kasvatusaltaita voidaan kayttaa, kuten jo nyt poikaslaitoksilla on, voidaan pelkalla selkeytysaltaalla pââsta hyviin puhdistustuloksiin kiintoaineen ja poiston suhteen ja melko hyviin tuloksiin mes ravinteiden poiston su hteen ( Liao 1970). Selkeytysaltaan rakennetta ja mitoitusta on kasitellyt mm. Warrer-Hansen (1982). Pintakuormateorian mukaisessa mitoituksessa tulee mitoitusvirtaamaa pienentââ kertoimella 1,5 turbulenssin eliminoimiseksi. Pintakuormateorian mukaisesti altaan syvyydellâ ei olisi merkitystâ altaan puhdistustehokkuudelta. Kuitenkin virtaus altaassa saisi olla korkeintaan 2-4 cm/s, mikâ voi asettaa joitakin ehtoja mes altaan syvyydelle ja viipymâlle. Vaakalaskeutusaltaan yleisinâ vaatimuksina on pidetty, ettâ leveys : pituus = 1:3 1:7, syvyys on 1,5 2,5 m, pintakuorma on s. 1,5 1,0 m/h, viipymâ on 1,0 1,5 h ja Re-luku (jatk. taulukoiden alla) 6 min 21 % 59% 11 min 15 min 30 min 29% 37% 37% 63% 66% 73% tusaltaassa kuvaa en rehutyypeillâ mes seuraava esimerkki (Warrer-Hansen 1982): tuorerehu puolikuiva kuivarehu 12% 19% 37% 15% 31% 37% 51% 59% _ 57% 66% _ 67% 72% on s 10 000. Altaaseen juuri ja juuri pidattyvan hiukkasen lakeutumisno peus saadaan karkeasti virtaamaan Q (m 3/s) ja altaan pinta-alan A (m 2) suhteena Q/A. Kuvissa 1 ja 2 on esitetty, mikâ merkitys kiintoaineen poistamisella on muiden kuormitusparametrien pienentâmisessâ sekâ millainen on kiintoaineen poistuman ja viipymân vâlinen suhde erityyppisissâ kasvatusaltaissa. Muirin (1982) mukaan tarvittava laskeutusaltaan kokonaistilavuus V, (1) saadaan selville laitoksen kalamââran S (kg), virtaamaan suhteutetun kalatiheyden P, (kg kalaa/1 H 20/min), altaan viipymân 0 (min) ja altaan tulo- ja poistovehykkeiden tilavuuskertoimien k ; ja k. avulla: V, = (SC) / P,) (1 + 1<, + k.). Altaan tarvittava viipyrnâ voidaan
80?1,' 6 0.34 2 40 4-, o '1'20 40 30 -Er 20 10. r","..,, 50 100 kiintoaineen poistotehokkuus (%) Kuva 1. Kiintoaineen poistotehokkuuden vaikutus muihin kuorrnitusparametreihin (Muir 1982). maa-altaat (Warrer-liansen 1979) Kuva 2. Viipymân vaikutus kiintoaineen poistotehokkuuteen (Muir 19e2). arvioida kiintoaineen laskeutumisnopeuden avulla. Tilavuuskertoimet voidaan arvioida sen perusteella, ettâ yleensâ altaan tub- ja poistovehykkeelle tarvitaan 1 5 % lisâtilaa tasaamaan virtauksia. Jos laitoksen vettâ ilmastetaan, voidaan kalatiheyttâ virtaamaa kohti lisâtâ ja vaadittavaa allastilavuutta pienentââ. Esimerkiksi, los P, kaksinkertaistetaan, niin V, putoaa puoleen. Toisaalta ilmastuksen seurauksena osa kiintoaineesta vol hajota, jolloin vastaavasti viipymââ altaassa, 0, tulee pidentââ. Jos laskeutuksessa kâytetaan lamellialtaita, jolloin laskeutumisala kasvaa kertoimella F., saadaan, ettâ MI 3 -N (Liao & Mayo 1974) (MAir 1978) 1311K (Muir & Lipper 1970) ;03 -N, PO4 -P (Muir 1978) kasvatussâiliftt (Muir 1978) /pyiireiit kasvatusaltaat (Warrer-Hansen 1979) 50 100 kiintoaineen poistotehokkuus (%) v i = (SO / P,F.) (1 + k + k.). Esimerkiksi, jos F, on 10 (10 lamellia), pienenee tarvittava ala 1/10:aan. Jos kiintoaine voidaan kerâtâ laitoksella pââvirrassa johonkin tiettyyn kohtaan, josta se voidaan poistaa sivuvirran avulla, voidaan tilannetta kuvata konsentraatiokertoimen F, avulla. Jos oletetaan, ettâ tâhân sivuvirtaan saadaan kaikki kiintoaine laitoksen poistovedestâ, niin V, = (SOF, / P,) (1 + k, + k.) Jos sivuvirta on 10 % pââvirrasta, niin F, = 0,1. Jos sivuvirtaan ei saada kaikkea klintoainetta, tulee altaan viipymââ lisâtâ ja kâsittelyâ laajentaa. On huomattava, ettâ klintoaineen poiston ensimmâinen vaihe ei juuri riipu kiintoaineen pitoisuudesta. Sen sijaan liukoisten aineiden poisto riippuu pitoisuudesta reaktion kertaluvun mukaisesti, mistâ seuraa, ettâ kâsittelyn vaatima ala, tilavuus yms. suurenee, kun pitoisuus pienenee. Pitkaviipymaisten kalanviljelylaitosten (viipyma tunteja) yhteyteen laskeutusaltaiden rakentaminen on kyseenalaista. Puhdistustehokkuus jââ tyypillisesti suuruusluokkaan kiintoaineella 10-15 %, kokonaisfosforilla 0-20 %, kokonaistypellâ 0-30 %, fosfaattifosforilla 0-20 % ja BHK:Ila 0-25 % (esim. Kekkonen 1982, Selânne ym. 1983, Pârjâlâ ja Tamminen 1984). Kalanviljelylaitoksilla on kâytôssâ monenlaisia laskeutus- ja selkeytysaltaita. Yksinkertaisimmat ovat pelkkiâ maapohjaisia altaita. Uutena sovellutuksena on viime vuosina esille tullut pyiferreselkeytin, jonka etuina ovat pieni tarvittava pinta-ala (n. 1/10 perinteisten selkeytysaltaiden vaatimasta alasta) ja suuri pintakuorma. Perreselkeytin soveltuu kuitenkin kâytettâvâksi vain hyvin laskeutuvalle kiintoaineelle (laskeutumisnopeus yli 0,5 0,6 cm/s) (Pârjâlâ ja Tamminen 1984). Hiukkaskooltaan pienen ja kevyen kiintoaineen, joka pyrkii menemâân perreselkeyttimen lâpi, poistamiseen perinteiset horisontaaliset ja vertikaaliset laskeutusaltaat ilmeisesti olisivat tehokkaampia, mutta ongelmana nâiden kohdalla on suuri tarvittava pintaala ja kalleus (Mashauri 1981). 3. SUODATUS 3.1. Turvesuodatus Turpeen suodatuskyky perustuu: mekaaniseen suodatukseen biologiseen suodatukseen pinta-aktiivisuuteen (adsorptio) kationinvaihtoon veden pidâtyskykyyn haihdutukseen. Turvesuodattimen mitoituksen ratkaisevat lâhinnâ suodattimen rakenne, turpeen kyky sitoa epâpuhtauksia ja turpeen vedenlâpâisevyys. Yleisiâ ohjeita turvesuodattimen rakentamisessa ovat (Selânne ym. 1983): 1) turvekerroksen paksuuden tulee olla 30-50 cm 2) turvekerroksen alla tulee olla salaojasoraa ja salaojat 3) lietevesi tulee johtaa tasaisesti suodattimelle jakolevyn tai settiitasanteen avulla 4) turve on tiivistettâvâ tasaisesti kostuttamalla, niin ettei suodattimeen synny oikovirtauksia ja ettei se kellu 5) altaassa tulee olla maareunat, jotta 12 Vesitalous 5/1984
turpeen kuivuessa reunaan ei synny rakoa, jolloin syntyy oikovirtauksia 6) suodattimessa tulee olla riittâvân korkeat reunat ylivuodon estâmiseksi 7) suotoveden laatua tulee pystyâ seuraamaan 8) salaojien on oltava tarpeeksi kaltevat, jotta ne eivât tukkeudu. Alustavana mitoitusarvona voidaan Selânteen ym. (1983) mukaan pitââ 200 g kokonaisfosforia/turverre. Turvesuodattimilla on saatu en kokeissa seuraavia puhdistustuloksia (Selânne 1982, Selânne ym. 1983): kiintoaine 40 99 % BHK, 62-91 % kok.-n 60-96 % kok. P 52-96 % PO4-P 39 97 % Turvesuodatinten pidâtyskykyyn vaikuttavat ratkaisevasti turvelaji, turpeen maatumisaste ja turpeen fysikaaliset ja kemialliset ominaisuudet. Suomen turpeiden kationinvaihtokapasiteetti on yleensâ 50-150 me/100 g ja vedenlâpâlsevyys 1o. ur cm/s (Seppânen ym. 1982). Kovin laajamittaista kâytânnôn kokemusta turvesuodatuksen mandollisuuksista ei viola ole olemassa. Menetelmâ sellaisenaan on kuitenkin kâyttôkelpoinen lâhinnâ vain pienillâ laitoksilla turpeen rajallisen pidâtyskyvyn ja rajallisen veden lâpâisykyvyn vuoksi. Turvesuodatus sen sijaan on kâyttelkelpoinen menetelmâ esimerkiksi selkeytysaltaista pumpattavan lieteveden edelleenkâsittelyssâ ja puhdistuksessa/tiivistyksessâ. 3.2. Mekaaninen hienosuodatus Yhdysva I loissa pyôrreselkeyttimen ohella hulevesien kiintoaineen poistamiseen on kâytetty mekaanista hienosuodatusta, joka kustannuksiltaan on osoittautunut vain hieman pyôrreselkeytystâ kalliimmaksi ja jonka sovellutusalue virtaaman suhteen on sama pyôrreselkeyttimen kanssa. Yleensâ mekaaninen hienosuodatin tai mikrosuodatin koostuu rummusta, joka on pinnoitettu joko suodatinkankaalla tai ruostumattomalla terâksellâ, jossa on 20-120 p.m reikiâ. Mikrosuodattimia on sekâ horisontaali- ettâ vertikaalisuunnassa pyôriviâ. Rumpu puhdistetaan vastapesulla, joka voi olla joko jatkuvaa tai jaksottaista. Drehwing ym:n (1979) tekemissâ kenttâkokeissa nâillâ suodattimilla pââstiin kiintoaineella 32-58 %:n puhdistustehokkuuksiin, kun samanaikaisesti perreselkeyttimen puhdistustehokkuus samanlaisissa olosuhteissa oli 18-55 %. Norjassa on alustavasti kokeiltu erilaisia rumpusuodattimia (reikâkoko 0,20 mm) poikaslaitoksilla. Alustavissa kokeissa puhdistustehokkuus on ollut 40-60 % (Lygren ym. 1983). 3.3. Hiekkasuodatus Pienillâ laitoksilla mandollisuuksien rajoissa saattaisi olla myôs karkea hiekkasuodatus, vaikka vaadittava suodatinala olisikin suuri ja suodattimen pesu ehkâ hankalaa. Suurilla laitoksilla ja kiertovesilaitoksilla ratkaisuna voisivat olla jatkuvatoimiset hiekkasuodattimet, jotka eivât tarvitse puhdistusta varten vastavirtahuuhtelua. Nâihin suodattimiin voitaisiin myôs yhdistââ kemiallinen saostus, ilman ettâ tarvittaisiin erillisiâ flokkaus- ja selkeytysaltaita. 4. POISTOVESI EN LAMMIKOINTI Jos kalanviljelylaitoksella on kâytettâvise paljon tilaa, niin erâs ratkaisu poistovesien kâsittelyyn voisi olla lammikointi, jossa vesi osaksi puhdistuu biologisesti ja osaksi mekaanisesti (sedimentaatio). Lammikon tâytyisi kuitenkin olla niin suuri, ettâ viipymâ siinâ huomioiden Suomen olosuhteet (alhainen veden lâmpôtila) ja poistoveden laimeus olisi useita vuorokausia (Liao 1979, Buhr ja Miller 1983). Lisâksi lammikko ei mielellâân saisi olla 40-50 cm:iâ syvempi. Kâytânnôssâ jârjestely vaatisi ilmastimella varustetut uoma-altaat, joissa vesi olisi jatkuvassa kierrossa. Lammikoinnilla saatettaisiin pââstâ parhaassa tapauksessa BHK,:n ja kiintoaineen suhteen yli 60 %:n puhdistustehokkuuteen ja myôs ravinteiden poistuminen olisi melko tehokasta. Talvella lammikoiden toimivuus voisi olla huono, mutta tâilôin vastaavasti kuormituskin on hyvin vâhâistâ. Pienillâ laitoksilla voisi mandollinen ratkaisu olla yhdistetty poistoveden lammikointi ja hiekkasuodatus. Karkealla 1 m:n paksuisella hiekkasuodattimella voitaisiin poistaa 0,1 0,5 kg BHK/m 2/d ja lammikoinnilla 0,02 kg BHK/m 2/d (Muir 1982). Hiekkasuodattimen tilavuus V, voidaan laskea kaavalla V, = SXK, / missâ 1_, = kuormituskerroin (esim. 0,1 0,5) S = kalamassa X = BHK:n tuotto K, = konsentraatiokerroin (ekvivalentti reaktionopeuden kanssa). Lammikoinnin vaatima tilavuus V, saadaan vastaavasti kaavalla V, = SXK, / L missâ L, = kuormituskerroin (esim. 0,02). Lammikoinnin ongelma on kuitenkin Suomessa epâedulliset lâmpôolosuhteet, ja kun puhdistettava poistovesi on laimeata, saattaisivat tarvittavat viipymât ja allaspinta-alat kohota epârealistisen suuriksi (Buhr ja Miller 1983, Middlebrooks ja Pano 1983, Uhlmann ym. 1983 ). 5. KIERTOVESILAITOKSET Yksi tapa ratkaista kalanviljelyn ympâristôongelmat hyvin tehokkaasti on slirtyâ kiertovesilaitoksiin, jolloin vesistôôn joutuva kuormitus kâytânnôssâ hâviââ lâhes kokonaan. Kiertovesilaitokset edellyttâvât kuitenkin useita tarkkaan suunniteltuja ja sââdettyjâ laitoksen sisâisiâ veden kâsittelyprosesseja. Nâissâ laitoksissa kierrâtettâvân voden kâsittelyyn kuuluu yleensâ orgaanisen aineksen poisto esim. selkeyttâmâllâ, flotaatiolla tai aktiivilietemenetelmâllâ, biosuodatin, jossa tapahtuu typpiyhdisteiden nitrifikaatio-denitrifikaatio, mekaaninen suodatin (esim. nopea hiekkasuodatin, painesuodatin, zeoliittisuodatin), veden sterilointi yleensâ UV-valolla ja mandollisesti myôs lâmpôtilan sââtô. Kiertovesilaitokset tarvitsevat yleensâ muutaman prosentin kokonaisvesimâârâstâ uutta vettâ kiertoon. Kiertovesilaitoksia ovat tarkemmin kâsitelleet mm. Hagebro ym. (1983) ja Nodal (1983) ja Jernelov ym. (1980). 6. SAOSTUS Poistovesien kiintoaine ja fosfori olisivat periaatteessa poistettavissa myôs saostamalla. Saostamisen kâyttôâ rajoittavat kuitenkin suuret vesimâârât ja alhainen kiintoaine- ja fosforipitoisuus. Saostuksessa kemikaaltkustannukset kohoaisivat suhteellisen korkeiksi. Lisâksi tarvittava saostusallas olisi suhteellisen suuri (ellei kâytetâ jatkuvatoimisia hiekkasuodattimia ohessa), koska alhaisen pitoisuuden vuoksi kontaktiaika saostuskemikaalin ja saostettavan aineen vâlille tulisi saada pitkâksi. Saostusta voitaisiin kyllâ ajatella yhdistettâvâksi tehostamaan kiintoaineon laskeutumista setkeytysyksikôissii ja edelleen tiivistâmâân Hatevettizi. TâIlaisia saostuksen sovellutuksia onkin jonkin verran tehty, joskin suurimittakaavaiset kokeet ja kokemukset vielâ puuttuvat. Saostuskemikaaleina tulisivat lâhinnâ kysymykseen FeSO 4, FeC1 3, Al (S0,) 3, Ca(OH) Ca0 ja polyelektrolyytit. Mm. Laukaan keskuskalanviljelylaitoksella tehdyt alustavat kokeet ovat Vesitalous 5/1984 13
antaneet hyviâ tuloksia (Selânne 1982, Selânne ym. 1983). Missâân kokeita tosin ei olla tehty kaytânnein mittakaavassa ja olosuhteissa. Saostuksen kâyttômandollisuuksia kalanviljelylaitoksilla vat kâsitelleet myôs Mâkinen ja Naukkarinen (1982). 7. ILMASTUS Erâânâ ratkaisuna poistovesien kâsittelyn osana on tuotu esille ilmastus. Jo nyt laitoksilla kâytetâân jonkin verran ilmastusta ja hapetusta, tosin kasvatusaltaissa parantamaan veden happitilannetta ja siten lisââmâân kasvua. Ilmastusta on esitetty toteutettavaksi mm. erillisessâ ilmastusaltaassa laitoksen loppupââssâ, johon vielâ voitaisiin lisâtâ fosforin saostus (Anonymous 1980). Ilmastus voitaisiin ajatella yhdistettâvâksi myôs laskeutuksen yhteyteen. Ennen selkeytystâ toteutettu ilmastus tosin voi vaikeuttaa muutenkin huonosti laskeutuvan kiintoaineen erottamista rikkomalla hauraita orgaanisia kiintoainehiukkasia pienemmiksi. Ilmastuksella voidaan vaikuttaa lahinnâ 02-, CO-, NO2- ja NH,-pitoisuuksiin. Ilmastuksessa 02-pitoisuus saadaan yleensâ helposti kohoamaan kyllâstysarvoonsa ja CO2-pitoisuus alenemaan. Tehokkaalla ilmastuksella kalojen târkein typpiaineenvaihduntatuote NH 3 saadaan hapettumaan mikâ jonkin verran vâhentââ sekundaarista hapenkulutusta vesistôssâ. Erityisesti maa-allas- ja maauomalaitoksissa tehokas ilmastus voi tosin edistââ esim. ja NH 4-:n liukenernista lietteestâ ja kiintoaineesta (Hollerman ja Boyd 1980). Ilmastuksen osalta tarvitaan vielâ tutkimusta sen taloudellisuudesta, tehokkuudesta ja kâyttôkelpoisuudesta poistovesien kâsittelyssâ. Joka tapauksessa erillisen ilmastuksen vaatima viipyrnâ olisi melko pitkâ (useita turtleja), jotta se olisi tehokas. Erilaisten ilmastinmallien tehokkuuksia standardiolosuhteissa lâhinnâ kalatuotannon lisââjinâ mat selvittâneet Heerfordt ja Nodal (1983). Ilmastuksen ja hapetuksen periaatteita ja erilaisia ilmastintyyppejâ ovat tarkemmin kâsitelleet Hiidenheimo yrn. (1983). 8. FOSFORIKUORMITUKSEN VAHENTAMISMAHDOLLI- SUUKSIA Rehun fosforipitoisuuden viihentiiminen 1,5 %:sta 1,0 %:iin merkitsee vesistôôn joutuvan fosforikuormituksen vâhenemistâ 45 %:11a, jos muut muuttujat pysyvât samoina. Vastaavasti rehukertoimen pieneneminen suodatinten pesuvesi r-e suodatin sââtôyksikkôlâmmitin painesuodatin tuleva vesi suodatin ilmasti n UV- - -..., -,c'-' t_ `W 1101.11 Ar - -2:k - _--7 ;----1,...,e, -tmer..0;.- IlleeP: ;,,,ill 4. e r 4r.aCr..1-7. 4 ' g- e - -- --1 I i it. ( der lor rq ip _...e - 1 iii Al 0.41..._.1,--.;.----_-4-=- --- :. _.,.. i ji crebe..., ----S--eatle&iq. lit %mime Kuva 3. Esimerkki kiertovesilaitoksesta. perreselkeytin poistuva vesi -...i.,...,..,, Taulukko 1. Esimerkki, 100 tonnia vuodessa tuottava laitos. laskeutus kalan fosforipitoisuus 0,4 % 0,4 % rehun fosforipitoisuus 1,5 % 1,0 % rehukerroin 2,0 1,5 fosforin talteenotto lietteenâ 0 % 50 % vesistôtin vuodessa joutuva fosforikuorma 2600 kg 550 kg Taulukko 2. Rehukilosta aiheutuva fosforikuormituslietteenpoiston en i tehokkuuksilla. fosforikuormitus g/rehukilo lâhde rehu lietteenpoiston tehokkuus 0% 50% 100% Selânne ym. T40 4,7 3,0 1,2 (1983) T40 5,9 4,8 3,7 Tess 5,1 3,8 2,4 ForeIli 6,7 5,4 4,1 Myllylâ 10,8 8,3 5,8 Selin ym. koerehu 5,7 4,1 2,9 kontrollirehu 9,7 7,1 4,4 Sumari ym. koerehu 10,1 7,0 3,9 kontrollirehu 8,1 6,3 4,4 1,5:stâ arvoon 1,0 pienentââ kâytettyâ rehukiloa kohti laskettua vesistôôn joutuvaa fosforikuormitusta 10-20 % ja tuotettua kalakiloa kohti laskettua fosforikuormitusta n. 45 % (Mâkinen 1983). Taulukossa 1 (Mâkinen 1983) on esimerkki rehun fosforipitoisuuden ja rehukertoimen merkityksestâ fosforikuormituksen pienentâmisessâ. Jos arvioidaan, ettâ puolet kokonaisfosforikuormasta on laskeutettavissa olevassa kiintoaineessa,riippuu kokonaisfosforin polstotehokkuus ratkaisevasti klintoaineen erotustehokkuudesta. Vaikka kaikki kiintoaine saataislin erotettua, mikâ edellyttââ, ettâ liete olisi tuoretta (ikâ aile viikko), jâisi vielâ puolet kokonaisfosforikuormasta jâljelle. Vanhentunutta, hajonnutta lietettâ poistamalla fosforikuormituksesta olisi parhaimmillaankin poistettavissa vain 20-25 %. Tâydellisellâ lieteenpoistolla fosforikuormituksesta pystyttâisiin poistamaan tapauksesta riippuen 30-80 % (Mâkinen 1983). Jos fosforinpoistossa pââdytâân lietteen erottamiseen, on samalla ratkaistava, miten syntyvâ lietevesi kâsitellâân. Lieteveden imeyttâminen maahan on vaikeaa, koska tarvittava maa-ala olisi yksi neliômetri/1,5-2,0 m3 lietevettâ (kiintoainepitoisuus n. 7 %). Pienillâ laitoksilla kâyttôkelpoinen menetelmâ voisi olla turvesuodatus. Turvesuodatuksessa yhtâ turvekuutiota kohti saa kokonaisfosforikuormitus olla korkeintaan 200 g. Kâyttôkelpoisin menetelmâ lienee, ettâ syntyvâ lietevesi pumpataan siiiiöihin, joissa siihen lisâtâân saostuskemikaalia, esim. kalkkia 2-4 kg/m 3. Tâmân jälkeen liete selkeytetâân ja siirretâân lietelavalle kuivumaan 1983). (Mâkinen 14 Vesitalous 5/1984