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Lamella dissolved air flotation treatment of fish farming effluents as a part of an integrated farming and effluent treatment concept a

Petri Jokela & Raghida Lepistö

a

a

Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland Accepted author version posted online: 01 May 2014.Published online: 09 Jun 2014.

Click for updates To cite this article: Petri Jokela & Raghida Lepistö (2014) Lamella dissolved air flotation treatment of fish farming effluents as a part of an integrated farming and effluent treatment concept, Environmental Technology, 35:21, 2727-2733, DOI: 10.1080/09593330.2014.919035 To link to this article: http://dx.doi.org/10.1080/09593330.2014.919035

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Environmental Technology, 2014 Vol. 35, No. 21, 2727–2733, http://dx.doi.org/10.1080/09593330.2014.919035

Lamella dissolved air flotation treatment of fish farming effluents as a part of an integrated farming and effluent treatment concept Petri Jokela∗ and Raghida Lepistö Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland

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(Received 7 January 2014; final version received 24 April 2014 ) Nutrient emissions from fish farming can be reduced by a bag pen, i.e., a floating circular basin which serves simultaneously both as a fish cultivation tank and a swirl separation tank. Solid matter (excreta and uneaten feed) is collected at the bottom of the bag pen and pumped as an underflow to a dissolved air flotation (DAF) unit for nutrient removal. DAF equipped with lamella elements was studied in real conditions. Altogether 3000 rainbow trout females (2.0 kg each) were cultivated. Solid–water mixture was pumped from the bottom of the bag pen to an equalizing basin using a sequence of 2-min pumping followed by a 4-min pause. In some tests the influent was pumped directly and continuously from the bag pen to DAF. The influent quality changed substantially: average suspended solids (SS) and phosphorus (P) concentrations were 290 mg l−1 ± 110 mg l−1 and 3.2 mg l−1 ± 1.2 mg l−1 , respectively. When the influent was fresh and P strongly associated with SS, DAF without precipitation chemicals produced up to 86% SS and 83% P removals. The influence of chemical doses was studied using 6.4–29.2 mg Fe l−1 with hydraulic loadings (HLs) of 11.0–11.7 m h−1 . SS and P removal did not change substantially and the effluent concentration levelled at 30 mg SS l−1 and 0.20–0.30 mg P l−1 , respectively. The lamella DAF, coupled with ferric precipitation, produced up to 90% P and 80% nitrogen reductions. HLs, excluding recycle water flow and lamella projection, up to 21 m h−1 could be used. Keywords: bag pen, dissolved air flotation, fish farming, lamella, nutrient removal

Introduction Fish farming requires large quantities of water to provide oxygen for respiration, to dilute the carbon dioxide and ammonium emissions, and to flush the rearing tanks. For example, water use in rainbow trout farming was more than 60 m3 kg−1 fish produced in a flow through system.[1] The main wastes from fish farming include excreta and uneaten feed. The fish excreta and feed are rich in phosphorus and nitrogen, which promote eutrophication of waters surrounding the farming sites.[2] Water management and effluent treatment concepts have been developed extensively for land-based systems.[3–5] However, for floating systems the number of concepts is still limited. Net cage system is commonly used for fish farming in lakes, rivers, and seas. A net cage comprises a floating collar from which hangs a net bag in which the fish are kept. Water moves freely in and out of the net cage, bringing in oxygen and removing the wastes to the surrounding environment. Consequently, controlling and limiting the release of wastes into the water environment is difficult.[6] In this study on fish farming, we replaced a net cage with an integrated farming and effluent treatment system to reduce nutrient emissions to the environment.

∗ Corresponding

author. Email: petri.jokela@tampere.fi

© 2014 Taylor & Francis

With a bag pen system, water is pumped tangentially into a circular floating basin, which serves simultaneously both as a fish cultivation tank and a swirl separation tank (Figure 1). The main water flow exits the pen through a vertical pipe in the centre, whereas solid matter (excreta and uneaten feed) is collected at the bottom of the tank and pumped as an underflow to the dissolved air flotation (DAF) treatment unit for further nutrient removal. The addition of the DAF stage is important to separate the solids and nutrients from the underflow and to reduce the volume of the sludge for disposal. One DAF unit can serve several bag pens. The controlled environment inside the bag pen makes it possible to increase the fish stocking density, thus helping to balance the increased operational costs compared to net cage farming.[7] The objective of this paper is to show, in real conditions, the applicability of lamella DAF to reduce nutrient emissions from bag pen fish farming. DAF was chosen as the solids treatment process due to its general amenability to high hydraulic loading (HL) and solids loading and the relatively good capability to cope with fluctuating conditions. We investigated DAF equipped with lamella elements as a means to increase HLs and thus reduce both costs and the footprint area required for the effluent treatment.

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P. Jokela and R. Lepistö

Figure 1.

Bag pen used for rearing of rainbow trout (volume 200 m3 , capacity 10,000 kg fish).

Figure 2.

Flowchart of the test setup.

Materials and methods The tests were conducted at a fish farm located in South West Finland archipelago, in the coastal area of the brackish Baltic Sea, using an integrated bag pen and DAF system (Figure 2). About 3000 rainbow trout females were cultivated in the bag pen. At the end of the DAF tests, the average fish weight was 2.0 kg and the total mass of fish in the bag pen was 6000 kg. The fish were fed with commercial pelleted feed (Aqualife 23, pellet size 8 mm, Biomar A/S, Denmark). The DAF pilot (KWI (UK) Ltd) had a rectangular flotation tank (length, 0.6 m; width, 0.5 m) equipped with three stainless steel lamella elements, inclined by 60◦ . The depth of the tank was 1.8 m and the volume 0.5 m3 . The lamellae were U-shaped: water flowed first upwards towards the open surface of the DAF tank, then downwards inside the U-elements and the clarified water was collected from the bottom of the U-elements (Figure 3). The lengths of the shorter and longer parts of each U-element were approximately 0.8 m and 1.2 m, respectively, and the total horizontally projected area of the U-elements was 1.5 m2 . The spacing between the lamella elements was 10 cm. HL was calculated using the surface area of the flotation tank

Figure 3.

Diagram of the lamella DAF tank. Not to scale.

(0.3 m2 ), but excluding the lamella projection and recycle flow. The influent flow was 1.0–8.8 m3 h−1 . DAF effluent was used for recycle water, which was saturated at a pressure of 550 kPa in a specially designed saturator (ADT). Sludge was scraped from the surface of the DAF tank by a scoop located at the far end of the tank. The DAF tank had a monitoring window that allowed observation of the U-elements. During the test runs, the settled solid–water mixture was pumped mostly from the bottom of the bag pen to an equalizing basin (2 m3 , slow mixing) on the adjacent shore using

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Environmental Technology a sequence of 2-min pumping followed by a 4-min pause. The influent to the DAF was pumped evenly from the equalizing basin. During some test runs, the influent was pumped directly from the bag pen to DAF bypassing the equalizing basin. The DAF was operated with and without chemical addition. During chemical addition we used a flocculation tank (volume, 0.215 m3 ; detention time, 3–9 min) equipped with a vertical shaft propeller (58 rev/min, four blades of 3 cm * 6 cm each). The flocculated influent was led to the DAF by gravity. Alternatively, when the influent to the DAF was pumped directly from the bag pen, the precipitation chemical was added into the influent pipe for pipe flocculation (volume, 0.4 m3 ; detention time, 3–5 min). Polyelectrolytes (polyacrylamides) were dosed immediately after the point of addition of the recycle water at the DAF unit. A mixture of ferric chloride and sulphate solution (127 g Fe3+ kg−1 ) was used for coagulation, except for one test run where polyaluminium chloride was used. All chemicals were from Kemira Chemicals, Finland. Multiple test runs with different detention times were performed with the DAF. For each run three consecutive samples were taken from the influent and the effluent. For a number of test runs also sludge samples were taken. Consecutive sampling was preferred to composite samples due to the highly fluctuating characteristics of the influent. Samples were stored on site at 4–5◦ C and analysed once a week for suspended solids (SS), phosphorus (P), orthophosphate (PO4 –P), and nitrogen (N) according to the standard methods.[8] All filtering (Whatman GF/A) was done immediately on site after sampling. Results and discussion Influent characteristics Due to the nature of fish cultivation, the influent quality changed substantially: the average SS concentration of the influent was 290 mg l−1 ± 110 mg l−1 (Table 1). For example, in one test run the influent SS concentration changed from 150 to 1200 mgl−1 and back with corresponding changes in influent P concentrations of 0.7 and 11 mg l−1 . Fish were fed periodically several times per hour with an automated feeder, causing peaks in loading inputs. During feeding, the fish changed their swimming routines and created currents, which had an effect on the movement of the solids at the bottom. In addition, the bag pen moved by the waves and the shaking effect was dependent on the roughness of the sea. Mechanical DAF treatment DAF tests without chemical addition were conducted at HL of 9.0, 11.0, and 21.3 m h−1 with 44%, 35%, and 19% recycle water addition, respectively. Influent SS concentrations varied both during and between the test runs (Figure 4). SS and P reductions varied between 46–86% and 20–83%,

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Table 1. Influent characteristics. Test run (N = 24). Temperature 16–17◦ C, pH 6.9–7.2.

averages

Average Std. dev. Std. dev. mg l−1 mg l−1 % Suspended solids (SS) Total phosphorus (P) Dissolved phosphorus (Pdiss) Orthophosphate (PO4 –P) Dissolved orthophosphate (PO4 –Pdiss) Nitrogen (N) Dissolved nitrogen (Ndiss)

290 3.2 0.79 2.0 0.59

110 1.2 0.21 0.69 0.19

38 38 27 35 32

9.4 3.9

4.5 1.3

48 33

respectively (Figure 5). Highest removals were achieved using the highest HL, when the influent had the highest SS, P, and N concentrations (Table 2). The influents for tests using HL of 9 and 21.3 m h−1 were pumped directly from the bag pen. The influents were fresh and the P was strongly (84–96%) associated with the particulate matter. Recycle flow of 19% was sufficient, compared to the results of the other test runs. In the test run with HL of 11.0 m h−1 , only 54% of P was associated with the particulate matter. For that test run the influent was first pumped to the slowly mixed equalization basin and then pumped to the DAF by a centrifugal pump. The longer (but still

Lamella dissolved air flotation treatment of fish farming effluents as a part of an integrated farming and effluent treatment concept.

Nutrient emissions from fish farming can be reduced by a bag pen, i.e., a floating circular basin which serves simultaneously both as a fish cultivati...
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