Air diffusers FAQ
1. Resistance of diffuser to fouling, carbonates, fats and bacteria
The air diffusers are a hybrid between solid stone type diffusers and a flexible membrane type diffuser. High levels of carbonates and iron present in the water can foul solid diffusers. If the water is very hard, with an alkalinity over 500mg/l as calcium carbonate, then the carbonate will tend to come out of solution and form a precipitate on the diffusers. The carbonate deposit will block solid type diffusers. Flexible membrane diffusers are resistant to this type of fouling but ghey are not suitable for membrane lagoons becuase they have to be anchored to the base. The Dryden Aqua diffuser is also resistant to this type of fouling because the surface of the diffuser is flexible, and hard carbonates will tend to crack off the surface. In particularly serious cases of fouling the diffuser are simply lifted out of the water and twisted by hand. If solid diffusers such as the stone type become fouled by carbonates they can become very difficult to clean, this is not the case with the Dryden Aqua diffuser.
The Dryden Aqua diffusers are often used for the treatment of water and waste water from sewage works, landfill leachate and industrial waste. In many cases the waste will have a very high fat content. In new effluent treatment systems the bacterial cell biomass may not have become established. In these systems there may be a reaction between the air and the fats to form hard fat deposits on all surfaces in the aeration tank, including the air diffusers.
A system will 100 air diffusers can be cleaned in a morning by one person by simply pulling the diffusers out of the tank by the the 1/2" delivery hose. Wearing a pair of gloves simply pull the diffuser through your gloved hand, and then through back into the tank. Once the activated sludge becomes established in the aeration tank, cleaning frequency may be reduced, however if the BOD / COD is over 1000 mg/l then we recommend this procedure every 1 to 2 weeks. In clean water system the diffuser cleaning frequency may be reduced to once every 6 months, when the differential pressure on the diffusers exceeds 4 psi, then they should be cleaned.
2. Does the diffuser need to be anchored to the floor of the aeration tank
The Dryden Aqua diffusers have their own internal ballast, which gives them sufficient weight to stay on the bottom of the aeration tank. Diffusers such as membrane type need to be anchored on to the base of the tank, and solid stone type diffusers need to be installed on network of pipes. The Dryden Aqua diffuser is therefore very easy to install. The internal ballast is composed of glass beads. The beads are very smooth and resistant to biofouling. The beads also move, and this again helps to keep the inside of the diffuser clean.
Our air diffuser is perfect for retrofitting in tanks or for supplemental aeration of existing tanks. In most cases where our diffusers have been used to support another aeration system, our clients often remove the original system and use the Dryden diffusers because they prove to be more efficient and much easier to maintain.
If the diffuser are more than 5m below the surface then the 1/2" delivery hose bouyancy may start to lift the diffuser. For deep tank or lagoon installations or where extra weight is required, additional ballast may be added.
3. Operation of the air diffusers, air flow rate.
The Dryden Aqua air diffusers can be made in lengths from 33cm to 300cm. The normal size of diffuser used for water and wastewater treatment is the 300cm long. This diffuser has a designed air flow rate of 9 to 12 cubm/hr of air. It is important to operate the diffuser at the designed flow rate. At a flow rate in excess of 9 cubm/hr the air escapes all around the diffuser, this minimizes the degree of fouling that the diffuser may experience.
The diffuser can be turned off and left in the wastewater, or it can be operated at the recommended air flow rate, however it should not be operated at a reduced air flow rate. If the diffuser was operated at less than 9 cubm/hr then the air would not escape all around the diffuser, and it would then be more prone to fouling by bacteria.
4. Recommendation air pressure.
The Dryden aqua air diffuser has been designed to operate with a high flow of air and low pressure drop. At the recommended air flow rate the pressure drop through the diffuser will be approximately 2 to 3psi. We normally use the air diffuser in tanks with a water depth of 2 to 5 metres. The hydrostatic pressure head at 5 metres water depth is 7.5psi. The pressure drop in the diffuser is 3psi, and the pressure drop on the pipework and hose will be around 3psi. Total pressure drop in the system is usually around 13psi in tanks with 5m water depth.
At air flow rate of 500 cubm/hr to 1000 cubm/hr of air we use 125mm diameter pipework for the air ring main, to minimize pressure drop. You should always use as large a diameter of pipe as possible in order to minimize pressure drop in the delivery pipe.
5. Size of air bubble
The oxygen transfer efficiency from air diffusers is a function of the bubble size from the diffusers. The smaller the bubble the better the transfer, reducing bubble size from 6mm to 3mm can increase the mixing of the water and oxygen transfer efficient by almost 5 times, bubble size is therefore very important. The Dryden Aqua air diffuser is one of the most efficient air diffusers available, with a bubble size around 1 to 3mm in most types of water. However the size of the bubble is a function of the quality and chemistry of the water.
In seawater, or water with a high ionic content, the bubble size can be smaller than 1 mm, as the water becomes more fresh, the size of the bubble will increase to around 3mm in clean freshwater. If the level of fats and lipids is very high, the lipids will tend to reduce water surface tension and increase the size of the bubble. As a guide in sewage treatment systems, bubble size normally ranges from around 2 to 5mm.
6. How much the air can be dissolved.
The table below provides a guideline for calculating the oxygen transfer of the Dryden Aqua air diffuser in water at different depth against the oxygen concentration already present in the water. The water type in question equates with sewage effluent to landfill leachate.
1 x diffuser code 3m long 10 cubm/hr will support 100PE population equivalent is sewage effluent treatment
1 x diffusers code 3m long running at 10 cubm/hr will dissolved into the water between 20kg and 50 kg of oxygen per day
Transfer efficiency, for 1 kw of energy expended 2 to 5 kg of oxygen will be dissolved into the water