Treating seasonal algal blooms
Seasonal algal blooms can cause major problems in water treatment works, particularly those where treatment is by direct filtration. Counter Current Dissolved Air Flotation (COCODAF) is now established as a high-rate process for the treatment of low-turbidity, coloured or algal-laden water writes Paterson Candy.
In the COCODAFF process, flocculated water is distributed evenly near the surface of the COCODAFF units by a system of submerged laterals and distribution cones and flows down through a rising air-bubble blanket to the filter below. The flow of flocculated water is counter-current to the rising air blanket, promoting greater bubble-particle interaction and enhancing floc capture efficiency compared to a conventional co-current DAF process. The micro-bubbles attach themselves to the floc particles and float to the surface. The resulting sludge blanket, supported by the air blanket below, is exceptionally stable and is removed readily by hydraulic means. Any sludge that is knocked down during desludging must pass through the air blanket again before reaching the filter media.
On an existing direct-filtration treatment plant, upgrading the process by the construction of a new DAF or COCODAFF plant may not always be feasible due to space constraints or the complexity of connecting with existing processes. The cost of constructing a stand-alone pre-treatment stage may also be difficult to justify. Using in-house computational fluid dynamics (CFD) and engineering expertise, Paterson Candy has adapted the COCODAFF technology for installation (retrofitting) into existing filter tanks. This means that existing direct filtration treatment plants can be upgraded to include the benefits of flotation without the need to construct additional process units. Installation of COCODAFF technology can enhance plant performance and in some cases may allow plant throughput to be increased.
Any dissolved air flotation process requires additional flocculation time, compared with a direct filtration process, necessitating the construction of new flocculators. For a conventional DAF process, the flocculators are typically sized to provide up to 30 minutes flocculation time. With COCODAFF, the flocculator volume is reduced to provide 15 minutes flocculation time as some flocculation is deemed to take place within the counter-current air blanket. For COCODAFF, Paterson Candy offers hydraulic flocculation yielding operational cost benefits and reduced maintenance requirements compared to conventional mechanical flocculators.
A major benefit is enhanced algae removal efficiency leading to maintenance of filter run times during seasonal algal blooms. Trials have shown that during algal blooms, a dual media COCODAFF process is able to maintain filter run times in excess of 24 hours, while corresponding dual media filters, operating in direct filtration mode and at reduced filtration rates are struggling to achieve run times of four hours. In this situation, there is a reduction in the amount of dirty washwater generated with the COCODAFF process and a saving in operating costs for the wastewater handling plant. The process offers operational flexibility. During periods of good quality raw water, the system may be turned off and the plant operated in direct filtration mode, minimising operating costs. During periods of seasonal algal blooms or increased raw water colour, the COCODAFF process can be quickly brought into service without interrupting the operation of the filters.
COCODAFF technology can be engineered to fit a wide range of filter designs. Conversion of existing filters to the process involves the installation of a new inlet channel which, depending on the layout and size of the filter, may be positioned across the centre or along a side wall. Inlet cones, the design of which have been optimised using CFD to maximise inlet distribution while minimising headloss, are arranged on a number of distribution laterals fed from the inlet channel. The recycle distribution system is suspended from the inlet distribution pipework.
To facilitate the hydraulic desludge, a flushing water channel and a sludge collection channel are installed along opposite walls of the filter. During a desludge, a sludge outlet valve opens and the sludge blanket is pulled over the desludge weir. Simultaneously, water fills the flushing water channel and overflows the ski-jump shaped weir, pushing the sludge blanket towards the desludge weir. The desludge is carried out automatically. The interval between desludges can be adjusted to suit changing raw water conditions.
The installation of COCODAFF technology to existing filters can enhance
process performance, particularly when algal blooms pose a problem, and lead
to a flexible and more robust treatment process solution.