Aeration with Ceramic Micro Bubble Diffusers
One of the oldest technologies known to mankind is the use of ceramics. For around 25,000 years ceramic clay materials have been taken from the earth, shaped, fired and used by humans for purposes of making pottery for storing food and water, ovens for cooking and melting metals to fashion into tools and weapons. Ceramic technology has advanced significantly over the past few decades.
The development of high temperature ceramics for space shuttle tiles, electrical insulators, bullet proof vests, artificial hip joints and superconductors for electronics all have made great impacts in advancing civilisation"s overall technology. Unfortunately, very little has been done in the past 30 years to improve traditional ceramic micro bubble diffusers for aquaculture.
Since the invention of a mechanical air pump in 1908 that opened the doors for the beneficial aeration in aquatic systems, traditional air stones were invented. These have been fashioned from limewood or porous stones.
Although fish farming has advanced in recent years and the most common methods used for aeration have been proven to be energy intensive (paddle wheels) or inefficient due to the formation of large bubbles as shown by porous hoses or even membrane diffusers, the costs of this equipment soon outweigh the long-term operational costs of wasted electricity and oxygen. These methods of aeration are unfortunately still popular and available due to misleading advertisements, insufficient infrastructure and possible lack of finance to upgrade to a modern practical technique.
In the 1980s, with advances in industrial porous ceramics for water filtration and catalysts, micro bubble diffusers were developed to address the need for more efficient and reliable aeration in fish farming. Fish stocking capacity is primarily limited by oxygen availability and therefore needs a reliable system to boost oxygen levels. These newly developed rectangular ceramic diffusers have traditionally been used in intensive fish farming to aerate water effectively during the transporting of fish, as a backup system when there is a pump or power failure, or simply to increase Dissolved Oxygen (DO) levels when required: e.g. during feeding or weather heat spells when fish require more oxygen and DO levels drop.
This new type of ceramic diffuser showed, in comparison to other methods of aeration, a very efficient way of increasing DO levels, but they lacked durability. Ceramics are very rigid and are therefore prone to breaking from over pressurising or accidental dropping during handling. Ceramic pores tend to clog up with algae growth over time thus decreasing performance. In addition, these diffusers show failure due to corrosion of the aluminium base and brass fittings, still being used to date to house the ceramic membrane for some popular diffusers designed over 30 years ago.
Unfortunately, over the past few years, some major brands of diffuser have given ceramics a bad reputation in the aquaculture industry. The quality of one brand deteriorated significantly since it outsourced its manufacturing. Another major brand, a cheaper version marketed by many corporate equipment suppliers, tends to break spontaneously. This has resulted in some fish farmers choosing alternative, somewhat controversial oxygenation methods.
Enviro Ceramic Diffusers were developed with a long lifespan and aeration efficiency in mind. In addition, it challenges the design of simple rectangular ceramic diffusers. Since 2010, Enviro Ceramic has continually studied and identified the key features required to optimise the efficiency of diffusers and addressed some of the main causes of failure during use.
Efficiency means reducing losses. Losses for diffusers are measured in oxygen consumption vs. maintaining safe DO levels for the fish and this is all measured in relation to energy costs. Diffuser performance optimisation can be achieved by tweaking the ceramic to produce the smallest possible bubbles and its overall design features.
These include: the smaller the bubble generated, the higher its capability for gas absorption into the water, efficiently increasing the DO levels. The bubble size depends mainly on the ceramic pore sizes, the number of pores per given area and their distribution over its surface and also on the surface tension between the ceramic surface and water interface.
"We found very interesting evidence on how porosity and its sizes as well as the hydrophilic nature of the ceramic influences the bubble sizes generated," says Kurt Holzträger, MD of Enviro Ceramic. "The surface tension ensures that the water shears off the newly formed bubble from clinging onto the ceramic surface. The faster it is released from the surface, the smaller it remains. This can also be observed with water side currents that greatly decrease bubble size".
"Another important factor is pore size: the smaller the ceramics pores are, the smaller the bubbles generated. We have analysed our ceramic compared to some competitor"s material with sophisticated porosity measurement equipment".
It was found that the ECD ceramic not only has a higher percentage of pores per given surface area, but also the smallest pores of 0.5 micron. The ECD not only generates smaller bubbles, but also generates an optimum amount of bubbles per area as it needs less pressure to achieve a given flow rate.
Measurement of bubble sizes is a very technical subject and currently no standards exist for the testing of diffusers" bubble sizes. The most common test method is with a high-speed camera and the resulting pictures of the bubbles are measured using sophisticated software to determine the sizes and distribution of the bubbles at various heights in the water. "We have analysed our bubble sizes with a high-speed camera and found them to be an average of 125 micron at a water depth of 40 cm. Unfortunately, the software could not determine bubble sizes under 50 microns as some bubbles will be in the Nano size range (under 1 micron). "New independent tests are being done with more accurate technology to determine our bubble size distribution and this will be published as soon as available".
Enviro Ceramic has moved away from the traditional rectangular design of the common diffusers available in the market. This rectangular shape is mainly used because it is easier to press the ceramic tile and huge volumes can be manufactured cheaply with little effort. Enviro Ceramic utilises a unique ceramic forming technique where complex designed technical ceramics can be cast to shape without the need for machining the ceramic before firing. "We have spent long innovative thoughts on our design and after many discussions came up with the doughnut design" – this novel patented design generates an air lift of water into the gas stream through the central hole. This concept moves anaerobic water from the tank floor to be aerated, increasing efficiency. This is something that the rectangular shapes can never physically achieve.
An important factor in increasing efficiency is having the right set up, following the recommended user instructions and specifications. The ideal installation for diffusers is to have good quality supporting equipment. An oxygen generator or oxygen bottles (for back up) with a pressure-reducing regulator feeds oxygen to a solenoid magneto valve. This valve is linked to a controller that has an optical oxygen sensor in the water monitoring DO levels. When the DO levels drop below the set value, then the controller sends a signal to open the solenoid valve and oxygen flows through the set flow meters to the diffusers. The diffusers will pump oxygen into the water and, once the required saturation has been achieved, the DO sensor signals the controller to switch off the solenoid valve. This keeps the oxygen at an optimum level, minimising waste and ensuring safe conditions for profitable fish farming.
Addressing Diffuser failure
Ceramics, like crockery, will break when dropped. Ceramic micro bubble diffusers need to be able to withstand tough conditions underwater and during routine maintenance. Various factors affect the life of a diffuser. "We have identified diffuser failure issues," Holzträger explains, "and are resolving them by applying resourcefulness to our product"!
Here a few examples: galvanic corrosion is an electrochemical reaction that occurs when two dissimilar metals make contact in the presence of an electrolyte: e.g. salt water. The most popular diffusers marketed to date are made with dissimilar metals. An aluminium housing is used with brass fittings for hose connectors. The aluminium is anodic and corrodes (even if coated with epoxy), while the brass acts as a cathode. This ensures that corrosion destroys the housing with time. Enviro Ceramic has addressed this by using only marine grade stainless steel fittings and a resin housing to overcome this problem altogether.
Clogging of pores. Microbes soon "nest" in the porous structure of a ceramic diffuser under water, especially when not in regular use. These microbes and algae soon block the ultrafine pores and fewer bubbles can form on the surface with time. Eventually the diffuser will show a drastic reduction in flow rate with the same amount of pressure used. Regular cleaning is required depending on the water quality. Cleaning ordinary ceramic diffusers is very labour intensive and the additional handling always has a risk of diffusers falling and breaking. Enviro Ceramic has addressed this issue by developing a novel inert ceramic that is anti-microbial, protecting the ceramic pores from within. This unique ceramic technology has shown that algal growth is reduced significantly on the surface, thereby considerably reducing cleaning maintenance.
Water hammer effect is when water is suddenly pressurised in a confined space and the resulting force can rupture its vessel. All ceramic diffusers are hollow inside in order to distribute the gasses evenly over the ceramic surface to achieve a uniform spread of bubbles. When the diffuser is not used, water seeps through the porous ceramic membrane and fills the hollow void inside the diffuser. Solenoid valves are commonly used to control the opening and closing of the gas supply to the diffusers
The sudden rush of the pressurised gas into the water filled diffuser"s inner plenum when opening the solenoid valve, can cause the hammer effect and crack the ceramic and/or its housing. If the ceramic is weaker than this pressure and hammer forces it will break. It is essential to use pressure regulators set to the diffuser manufacturer"s recommended setting and employ an emergency over-pressure release valve. Farmers now tend to purge the diffuser every few minutes, for just a few seconds, to keep the water out. This method is easily set to run automatically with a decent controller." We have however found some customers not using this technology and therefore we are in the process of developing a novel technique to overcome these failures caused by over pressurizing and the water hammer effect".
As for the dropping of ceramic diffusers is concerned, they should be "handled with care" at all times and they will last for a very long time! Our latest model is fitted with silicone "feet" that have a bounce effect when the diffuser is dropped; they are also more stable on the tank floor from moving around if not mounted on a frame.
We aim to continuously improve our diffusers and are working with some leading universities and technology companies as well as fish farms around the world to maintain hands-on innovative solutions.
Our world"s first ozone resistant ceramic micro bubble diffuser is now in production. This new diffuser is manufactured with 100% ceramic materials that are inert to the effects of ozone. This technology opens the doors to new applications previously not possible, due to the lack of efficient ozone transfer capabilities of existing coarse bubble air stones. This diffuser is robust and makes the smallest bubbles possible under 1 bar pressure! Check the company website for more on this development and other improvements in the near future.
We are utilising modern ceramic technology and science to maximise the diffuser capabilities to the next level. Our aim is that customers should have "no argument" as to what aeration system should be chosen. With support by aquaFUTURE e.K. our products are successfully marketed internationally and supporting agents are available in various regions close to you.
By Dietmar Firzlaff, aquaFUTURE, USA