Reduction of nitrates

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Reduction of nitrates

Postby prussell » Wed Oct 16, 2013 5:26 pm

I believe that I have developed a novel method of controlling nitrates deploying the old technology of reverse flow undergravel filtration. Undergravel plates have fallen out of favour recently as it is argued that they trap detritus thereby increasing the production of nitrates, they do not facilitate aquarscaping and are not conducive to growing plants.

I siliconed a strip of glass about 5cm high across the floor of my 130 litre aquarium creating two separate floor areas. One section comprised of about 30% of the total floor area of the aquarium and the other is approximately 70%. Filter plates were laid in both sections and each plate has its own downflow tube. The flow of water is delivered down both tubes and forced up through the substrate by an external canister filter. The canister is filled with several layers of media chosen to provide increasing degrees of mechanical filtration before the water is returned to the aquarium. The smaller of the two filter plates has a substrate depth of 6cm and the larger one has a covering of 9cm

The outlet pipe of the canister filter is divided into two and the flow rate regulated so that the vast majority of the flow is directed up through the substrate of the smaller section. The larger section has a regular drip totalling about 15 ml/min
The intention is to keep the smaller section aerobic so that bacterial oxidation of NH3/NH4+ and NO2 occurs, and the larger section anaerobic so that facultative anaerobes reduce the NO3 to nitrogen. Biodegradable ‘Deniballs’ were pushed into the substrate on the anaerobic compartment to provide a carbon source for the bacteria. A small quantity of ‘Denimar’ powder in suspension is also administered to the down tube of the anaerobic compartment daily.

Following a period of maturation, my aquarium was maximally stocked with community species, mostly tetras, in soft water. My weekly tests indicated that ammonia and nitrites were below detectable levels and nitrates were usually between 15-20 ppm. This value is substantially lower than that previously achieved by weekly water changes of twenty per cent

During the first three months of the trial of the ‘dual flow’ undergravel system, the aquarium was decorated with silk plants. I was aware that some authorities advocate the use of a heated cable in the substrate to create a micro circulation around the roots of plants so I reasoned that plants may well flourish in the 'slow-flow' substrate section. I increased the intensity of the lighting and introduced CO2 before planting. I grow plants that attach to wood or rocks on the ‘fast flow’ side. The plants are growing vigorously and their introduction has caused the nitrates to fall further to a typical value of less than 5ppm

I use the ‘dual flow’ system to introduce carbon dioxide. CO2 cannot just be bubbled it into the water because the bubbles rise too rapidly and little gas is absorbed. More importantly, it then forms a blanket over the surface excluding the lighter oxygen with potentially disastrous results. I introduce CO2 by placing an air-stone into the ‘fast-flow’ down tube where the bubbles are swept down under the gravel by the fast flowing water and trapped there giving them time to be absorbed. The ‘slow-flow’ down pipe is used in a similar manner to deliver fertilizers directly to the plants’ roots which is beneficial to hungry root feeders such as Amazon swords.

Since using a dual flow undergravel system, I only change approximately 10% of the water once a fortnight and I have no problem with algae.

Before setting up the dual flow system, I had some concern that the anaerobic section may produce hydrogen sulphide. However, I reasoned that the dual-flow system was, in principle, similar to the deep sand beds that have been used by marine reef keepers without problems. Secondly, anyone relying on an internal or external canister filter for biological filtration will also have a substrate containing an all but static volume of water. In the early stages of the trial, I tested the water from the ‘slow-flow’ section by pushing a large syringe into the substrate and withdrawing a sample. I always found the sample to be murky but sweet smelling. Finally, and the real proof of the pudding, my dual-flow system has been running in excess of six months; I have ideal water quality and my fish and plants are in rude health.
prussell
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Re: Reduction of nitrates

Postby prussell » Sun Apr 27, 2014 9:13 am

I have made two modifications to the operation of the dual flow under gravel system I installed in my aquarium over a year ago.
Firstly, I no longer use ‘Deniballs’ pushed in to the substrate as a carbon source for the anaerobic denitrifying bacteria. I discovered that the Deniballs fragments after several months of use. This did not cause a major problem but it turned the aquarium into a snow dome whenever the substrate was disturbed during routine maintenance. I now use glutaraldehyde which is sold as the so-called liquid carbon to promote plant growth. I administer small diluted quantities to the downflow pipe on the slow-flow side three times a day. I ensure that the total dosage is below that recommend in its normal use so the there is no possibility of overdosing. Any glutaraldehyde that passes through the substrate un-metabolised will be assimilated by the plants. If required, a dosing pump could be used during holiday periods.
The second modification involves the delivery of water to the slow-flow side. I used to have a tee -piece on the outflow on my canister filter and controlled the flow-rate with a 6mm airline valve. Biofilm forming in the pipe used to break free partially blocking the valve causing the flow rate to drop. Therefore it was necessary to open the valve fully each day and re-set the flow. Although this was neither a time consuming nor an irksome task, it became a problem when leaving the house for a few days because the flow would stop completely due to a blockage of the valve. I now direct all of the output from the canister filter down the fast-flow side and I have made a very simple air driven up-lift tube to deliver water into the slow-flow tube. It is easy to regulate the flow of water by controlling the airflow with a 6mm air valve. Once set at 15-20ml/minute, the flow-rate remains constant for very long periods. Apart from a flush of the up-lift tube every few weeks, the system is maintenance free.
I continue to perform tests for metabolites on a weekly basis which demonstrate that ammonia, nitrites and nitrates are consistently below detectable levels.
prussell
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Posts: 19
Joined: Sat Mar 23, 2013 10:57 am


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