Saturday, July 23, 2011

Grouper Farming Using RAS - Calculation 3

So long I was not able to update my blog. I was stuck with repair and maintenance works at my small project. Anyway, as promised we should carry on with another aspect of our RAS, i.e. to calculate the ammonia production, biological oxygen demand (BOD) and suspended solid (SS).

First, let we look at the food that we are going to fed our grouper fishes. Normally, if we are using commercial feed, we will know the level of protein content in the feed. The manufacturer will be proudly mention this fact on their packaging. Let say it was 40% protein feed that we are using to fed our grouper fishes. The calculation for ammonia will always be based on the nitrogen (N) availability in the protein. Based on molecular weight of the protein, we know that there are 16% of N in the protein in the feed given to the fishes. So, in 1 kg of feed, there are 64g of N (40% of protein/kg of feed x 16% of N in the protein).

If we know the body profile of our chosen species, we might know their protein content per kg of fish. Please do some literature review of your species to get this fact. In my case study, it was 20% protein in my grouper. Based on food conversion ratio (FCR) 1:1.5, one kg of feed will give 666.67g of fish weight. Therefore, per kg of fish, there will be 133g of protein (666.67g x 20% of protein/fish of 666.67g weight). Using the previous protein molecular weight, the N will be 16% of the protein, we will know that our 666.67g fish will have 21g of N inside their body (16% of N in protein formula x 133g of protein retain in the fish body). Therefore, our grouper fishes actually retain 21g of N from 1kg of feed that contain 64g of N. The balance 43g of N will be excreted out (64g in the feed - 21g retain in the body). So, we now may able to know the weight of ammonium produced / kg of feed by multiplying the amount of N excreted (43g) with 0.8 and we get 34g of ammonium produced per kg of feed fed to our fishes. In maximum standing stock (MSS) situation, the ammonium produced will be 3224g (based on maximum food given of 94.43kg) per day while in Average Standing Stock (AvStS) situation, the ammonium produced will be 2728g (based on 79.95 kg feed given daily). The amount of ammonium produced here is our concern and this is the burden that our bio-filter must solve before the water re-enter the farming tanks.

The biological oxygen demand (BOD) normally will be 500g for 1 kg of feed given. Therefore, in MSS situation, 47 kg (94.43 kg feed given x 500g BOD/kg of feed) and in AvStS situation, it will be 40 kg (79.94 kg feed given x 500g BOD/kg of feed). Based on experience, 250 gram of suspended solid will be produced from 1 kg of feed, so in MSS situation, our system will produce 23.61 kg of suspended solid per day (94.43 kg of feed given x 250g of SS/kg of feed) while in AvStS situation, it will produce 20kg of suspended solid per day (79.94 kg of feed given x 250g of SS produced /kg of feed given).

So far, we already able to calculate the consumption of oxygen and production of  carbon dioxide,  ammonium - N, biological oxygen demand and total suspended solid from our system. From these information, we will be able to calculate the design peak load of the system which will be used to determine the size of our bio-filter and maximum flow rate required to do the cleansing of our system.  I will try to discuss these aspects in my future posting.