FIT Systems FAQ and Troubleshooting

1. What is causing my high nitrite (NO2) levels?

•  Low dissolved oxygen: Elevated levels of Nitrite may occur if the dissolved oxygen concentration in the effluent leaving the filter drops below 2 mg/l. Low DO concentrations leaving the filter can often be solved by increasing the dissolved oxygen levels in the tank/pond or through increased aeration or by increasing the flow rate through the filter.
• Low alkalinity: Elevated Nitrite levels may also occur if your total alkalinity (as CaCO3) drops below 80mg/l. We recommended you maintain your alkalinity at 100-200 mg/l as CaCO3 at all times. If you experience low alkalinity, simply add baking soda to the filtration system periodically to maintain proper levels. Sodium bicarbonate is not suitable for aquaponics applications because sodium can harm plant development and growth. Applications of different liming agents, such as potassium hydroxide (KOH) and Ca(OH)2 are recommended for balanced plant growth.
• Too much backwashing: Elevated Nitrite levels can also occur from over washing the bead bed. If the flow rate, effluent oxygen and alkalinity are satisfactory, the backwash frequency can simply be reduced. This situation typically occurs when you go from periods of high loading and frequent backwashing to periods of reduced loading but still maintain frequent backwashing.
• Coldwater system backwash interval: Although generally the nitrification can be achieved across a wide ban of backwash intervals, backwashing for cold water systems must be more carefully managed. Generally, backwashing should be limited to a low frequency (<1 day) for cold water applications. The reduced frequency allows more time for the slow growing NOBs to recover from backwashing biofilm damage.

2. My effluent has a really low dissolved oxygen level. Why is this happening and how do I fix this?

• Too Low a flow rate: Low Effluent Dissolved Oxygen concentrations are usually the result of too low a flow rate. Effluent D.O. concentrations should be maintained above 2 mg/l at all times. If you are not flowing water through the filter at the filters maximum flow rate, simply increase the flow through the filter.
• Increase Aeration: Or you can increase the amount of aeration in the tank or pond to increase influent D.O. concentrations which will usually result in increased effluent D.O. concentrations.
• Too low a backwash frequency: Low DO concentrations may also occur if the backwash frequency is set too low. If the bead bed is allowed to clog, reduced flow and effluent oxygen concentrations will occur, which will affect the nitrification performance. Please refer to “Backwash Frequency” section for recommended backwash frequencies at various feeding rates.
• Failure to remove sludge in a timely manner: Low D.O. concentrations can also result if you do not remove sludge and waste from the filter often enough. The sludge should be drained every week minimum, to prevent excessive oxygen consumption by the activity of heterotrophic bacteria.
• Low Dissolved oxygen in the fish tank: Low dissolved oxygen levels in the filter effluent can also be caused by low dissolved oxygen in the influent (or fish tank). Filter sizing is generally based on the assumption that the influent dissolved oxygen concentration is between 5-6 mg/l. Increase the aeration in the tank or consider operation the filter in a recirculating format with a well aerated sump.
• Bacteria consuming all oxygen, increase backwash rate: In the some application the organic content (BOD) of the influent water is so high that the heterotrophs consume all the oxygen. In this case, the backwash frequency can be raised to increase the flowrate through the filter as the biofilm is more quickly removed. This procedure reduces the filter’s organic loading because the bacterial biofilm is removed before it can fully respire. Filter oxygenation is improved by the higher flowrate.

3. Why do I have a reduced flow rate/ pressure loss?

• Clogged bead bed, turn up the air: The most common cause of a drop in flowrate through a Polygeyser is clogging of the bead bed because the backwash frequency is too low. Clogging can occur because of an accumulation of suspended particles or from excessive biofilm growth. To overcome this problem, simply increase the air flow to the charge chamber to trigger a more frequent backwash cycle.