Sludge production and utilization
Removal of particles from the water creates sludge comprising water and particles. The water content and dry matter (DM) or total solids (TS) in the sludge depend on the particle filter used. To give an idea of the amount of sludge (faeces) created by the fish, the following estimate can be used: for each kilogram of feed eaten by rainbow trout 20% faeces are produced, measured on 100% DM basis.40 This value, however, depends on a number of factors including feed composition, feed type, fish size and fish species, and will therefore vary. If the feed con-version rate is above unity and traditional dry feed is used, there will be feed loss that goes directly to the outlet in addition to the sludge produced from the faeces. This also shows the importance of correct feeding and avoiding feed losses.
How much sludge is produced per kg commercial dry feed supplied to rainbow trout if the purification efficiency of the filter is set to 50% measured as DM? What happens if the feed conversion rate increases to 1.2?
Per kg feed supplied, 200 g of sludge is produced and the filter collects 50% of this. This means that the amount of sludge collected per 1 kg feed supplied is 100 g.
If the feed conversion rate is 1.2 only 0.83 kg of the 1 kg feed supplied will be utilized for growth, while the rest will be feed loss. Calculating that 20% of the feed eaten is converted to faeces, this represents 0.17 kg. If this is added to the feed loss, the amount is 0.17 + 0.17 = 0.34 kg. If collecting 50% of this, the amount collected per kg feed supplied will be 170 g.
The actual amount of sludge can be much higher; the percentage DM in the collected sludge depends on the filter system used. In a mechanical filter it is mainly a consequence of how the straining cloth is back-washed, whether using air or water, and if water, the amount. Normally the percentage DM in the sludge is 0.1–1%, but in special filters it might be up to 5%. It is advantageous to have as much DM in the sludge as possible, which means that back-flushing with water is disadvantageous. The only reason for back-flushing with water is that it is an effective system. A large proportion of DM in the sludge reduces the amount that must be further treated and transported; the sludge is therefore often dewatered to increase the percentage DM. Filter presses and special centrifuges can be employed for dewatering.16,23 The sludge may also be sent to a settling system for further separation of particles.26 Vertical sedimentation in a cone has been used and increased the DM in the sludge to 7–10%.
Normally the sludge must be stored for a period to accumulate enough so that it can be collected economically. A particle removal system will there-fore include storage tanks for the sludge.
To make it possible to store the sludge, it must be stabilized. If the sludge is immediately placed in an open container with access to air at the surface, an uncontrolled decomposition process (rotting) will take place. This will smell and have negative effects on the development of bacteria and the content of nutrients. Subsequent use of the sludge can be inhibited because of this. Correct decomposition makes the nutrients in the sludge available for plants so that it can be used as fertilizer on agri-culture land. Sludge from fish farms is rich in organic nitrogen (3–9% of DM) and phosphorus (1–4% of DM). In addition, the concentration of heavy metals is usually below regulatory limits.41This makes the sludge useful as a fertilizer.
Untreated sludge may contain pathogenic (negative) micro-organisms such as viruses, bacteria and parasites.42When infected sludge is spread on agricultural land and there is drainage to lakes or rivers pathogenic micro-organisms could be transferred to the local fish strains. Birds could also transfer pathogenic micro-organisms from the sludge to lakes. Therefore the sludge must be treated to inactivate the negative micro-organisms before it is spread; this is not achieved with uncontrolled decomposition.
There are several ways to inactivate pathogenic micro-organisms in the sludge; wet or dry com-posting is commonly used. Another method is to add lime to raise the pH in the sludge and hence inactivate the micro-organisms. Both these methods will also stabilize the sludge so that it can be stored, and make it suitable for use as fertilizer on agricultural land, a very important use for sludge, normally a good fertilizer because of the high nutrient content.
When composting sludge, controlled aerobic decomposition occurs. Due to the low content of DM, wet or liquid composting is employed for fish farming sludge (Fig. 5.12). Before composting the
sludge may be mixed with manure or municipal sludge. In a community there may be a centrally installed reactor. The sludge is poured into a con-tainer where air is added, for instance through an injector pump. In addition the sludge is circulated around in the tank so air comes into contact with all the sludge. This results in controlled bacterial development in the container which decomposes the organic matter. The process is thermophilic, so the temperature increases depending on the sludge. For fish farming the sludge is energy-rich, experimentally producing 3.1 kWh/kg DM,44 and the temperature rises to 60–70°C (Fig. 5.13). This high temperature is maintained for some time so that the pathogenic micro-organisms are inactivated and the sludge is stabilized for storage. To get rid of the smell developed during the composting process it is an advantage to include a smell filter, for instance made of peat, through which all gases emitted by the composting process have to pass.43–45 Overall, the composting process will result in biological stabilization of the sludge, removal of the major odour compounds, increased availability of some plant nutrients, nitrification and denitrification, and improved waste consistency.
If the process is run without access to oxygen, anaerobic digestion, also known as fermentation, will occur. As for composting, naturally occurring micro-organisms in the sludge are utilized. It is important that no air is supplied, so anaerobic degeneration of the organic substances will occur. Normally some heating of the sludge will be neces-sary to allow the micro-organisms to develop. The sludge is stored in a closed digester during this process. For animal manure it takes 10–35 days; a high fat content reduces the time necessary. This fermentation produces methane gas; the process is therefore also called biogas fermentation. In addi-tion to the production of biogas, there will also be a reduction in offensive odours, a breakdown of organic mass, a reduction of pathogens and an improved fertilizing value due to easier availability of the nutrients.42The biogas can be used for heating or electricity production, so there is actu-ally a positive output from the process. Sludge that has gone through this process can be stored for later use as fertilizer for agricultural land.
Adding lime (CaO) or slaked lime (Ca(OH)2) to the sludge to increase the pH, will also stabilize and disinfect the sludge. In experiments with sludge from fish farming it was shown that by increasing the pH to 12 and maintaining this value for 7 days, more than 99.9% of the pathogenic viruses and bacteria were killed.41The sludge produced is well suited for use as organic fertilizer.
When establishing a system for particle removal it is therefore necessary to think not only about the particle filter itself, but also sludge production and its utilization. This includes tanks for sludge collec-tion (Fig. 5.14).