Design of Submerged Fixed Bed Filter for High Density Shrimp Culture in Recirculating Aquaculture System

Shrimp is Indonesia’s the leading export commodities from Indonesia, but in recent years the shrimp industry has been threatened by disease outbreaks due to environmental quality decline. To overcome these challenges, there is a need for a cultivation technology which provides opportunities for intensive shrimp culture in a narrow area, resulting high productivity and having less environmental impact. Recirculating aquaculture systems (RAS) was the best option to provide high density shrimp culture without leading to environmental damage due the system equiped with water treatment unit. This study aimed to design a submerged filter which can be used in commercially recirculating systems for high density shrimp culture. Submerged filter as water treatment unit for ammonia conversion, it was designed based on organic load mass balance derived from feeding regimes. A RAS with 8 m3 volume capacity use for rearing 400 shrimps m−3, has dissolved oxygen levels range between 6-7 mg L−1, nitrification rate 0.45 g TAN m−2day−1 and TAN levels <1.8 mg L−1, based on spreadsheet calculation will require submerged filter media with 0.07 m3 in volume, media depth 0.3 m, and filter media specific surface area 1200 m2 m−3. Submerged filter design for high density shrimp culture in a narrow place must have a compact design. It can be achieved by selecting filter media type with large specific surface area values. Filter media with a large specific surface area makes biofilter design has smaller dimensions. Submerged filter as water treatment unit in recirculating system for high density shrimp culture will reduce TAN level in water. Waste water in this system is reuse for shrimp culture, no waste water disposal to the environment which leading to sustainable aquaculture practice.


Introduction
Recirculating aquaculture system (RAS) is using reused water during fish farming operations. This method is more environment friendly because it uses water treatment units and consume less raw water than fish farming with a flow through system. Application of water treatment unit such as biological and physical filter become the core of RAS technology, each filter can be adjusted to meet water quality requirements for aquaculture commodities [1]. RAS technology uses recycled water during the production process, leading to non waste water discharged into rivers or sea.
Waste water resulting from intensive shrimp farming usually contain high level of organic content, which derived from accumulation of organic residues from uneaten feed, ammonia excretion, feces, and feed particles [2]. Untreatment waste water from intensive shrimp farming which streamed directly into the sea will cause an environmental impact [3]. Application of RAS technology provide opportunities for intensive shrimp farming to produce a large quantity of shrimp with less severe environmental damage and more sustainable.
RAS technology was not commonly use for intensive vaname grow out. Farmer prefer use RAS technology for shrimp nursery because this system is able to manage more stable water quality. The aims of this study is to design submerged fixed bed filter as one device uses in RAS water treatment unit, which can be use in commercially viable recirculating systems for high density shrimp culture until reach harvest size. Submerged fixed bed filter design formulation in this study applied from [4,5] research, we compiled them in computer spreadsheet to calculate filter dimension.

Filter Design
Design of recirculating system filter based on the highest waste result during farming operation which achieved at harvest time [5]. In this study we will design submerged fixed bed filter for recirculation system with total volume 8 m 3 , it can be use for shrimp farming with density 400 shrimp m -3 , and total harvest 46 kg of shrimp. Filter capacity is designed to be 1.5 times greater than than total harvest weight to prevent water overflow. Water quality requirements for shrimp farming are: dissolved oxygen range between 6-7 mg L -1 , TAN <1.8 mg L -1. Assumptions use in this study are: Feeding rate 3% of body weight, feed protein content 30%, filter efficiency for TAN removal 50%, nitrification rate 0.45 g TAN m -2 day -1 , specific media surface area 1200 m 2 m -3 . Equation (1) until equation (7) used to calculate the biofilter dimensions, all equations adopted from [4,5].

Experimental Design
Recirculating aquaculture system (RAS) used in this study has eight culture tank, each culture tank has 1000 L in volume,1 unit fixed bed submerged filter with dimension presented in (Table 2) filled with sponge media and one 1766 L aeration tank presented in figure 1. Shrimp with average weight 0.74 + 0.02 gr, reared in each culture tank with density of 400 shrimp/m 3 for 60 days. Shrimp were fed five times a day with CJ shrimp feed (protein content + 32-35) with feeding rate 3% of total shrimp weight. RAS is operated for 24 hours with zero sea water replacement, however we add fresh water periodically to keep water salinity beyond 23-25 ppt.

Water sampling
(1) The Total ammonia nitrogen (TAN), ammonia, nitrite and nitrate from all rearing tank were recorded every 10 days by composite sampling method. Water salinity, dissolved oxygen, temperature, pH in each rearing tank and filter effluent pipe, were recorded daily for 60 days with YSI 556 multi parameter tester.

Result and discussion
3.1. Submerged fixed bed filter design Submerged fixed bed filter used for this study was built with some assumptions and criteria presented in Table 1. The calculation result of formulation (1) until formulation (7) to define filter dimension was presented in Table 2.  Table 2 showed that fixed bed filter dimension used in this study has 0.3 m media depth, 0.543 m bottom area diameter and 1200 m 2 /m 3 filter media surface area. Filter media play important role to determine filter dimension and influence filter ability in reducing ammonia concentration. High surface area filter media will result in smaller filter dimension. [6] compared ceramsite, zeolite and sponge filter media performance for municipal wastewater treatment, he stated that sponge filter media has better ability as biofilm carrier than other filter media and resulting higher capability in TAN removal. Based on his research, in this filter design we used sponge with 1200 m 2 /m 3 surface area as filter media. Aside from media filter selection, feeding frequency also affecting waste production 4 variation in recirculation system. Feeding mechanisms difference will impact flow rate design, fluctuation of water quality and alteration on water treatment unit dimension required for recirculation system [7].

Water quality parameter
The result of RAS water quality values during the study was compared with the water quality standard requirement for the grow out culture of white shrimp (Table 3). Figure 2 shows the water nutrient concentration in the rearing tank during the 60 day culture of white shrimp.   Water quality parameters have a great impact on crustacean immune system [10] . Moreover water quality degradation can lead to shrimp growth disruption which resulting lower survival rate [11]. Application of fixed bed submerged filter in this recirculating system is essential to water replenishment and comply with water quality requirement for optimum shrimp growth. Table 3 showed that fixed bed submerged filter has good performance in keeping nitrogen value below standard level. Furthermore, all water parameter in the recirculating system meet water quality standard for vaname growout. [12] also conducted research on shrimp growout in recirculation systems, their system reach maximum ammonium concentration 1.8 ppm, nitrite 3.7 ppm and nitrate 170 ppm, compared with their system, water quality result from this study has better performance.

Conclusion
Submerged fixed bed filter design dimension strongly influenced by the highest amount of waste generated in the system as well as the desired water quality criteria. Moreover, if the high density shrimp culture located in narrow place, the filter design must has a compact design. Smaller and compact filter dimension can be achieved by selecting filter media type which has large specific surface area values (example sponge media). Submerged fixed bed filter as water treatment unit in recirculating system for high density shrimp culture will reduce TAN level according to desired criteria. Waste water in this system is recycle and reuse during shrimp farming, zero waste water disposal to the environment which leading to sustainable shrimp farming method.