The new 17,000 m3 Digester 6 at Changi Water Reclamation Plant will be mixed using a custom Rotamix pump and nozzle mixing system.
With a diameter of 32 metres across, a sidewall height of 13.9 metres and a cone depth of 18.3 metres, Engineers working on the Digester 6 project at Changi WRP were presented with some unique challenges. One major consideration was selecting a sludge mixing system that would be capable of mixing such a large volume digester and deep cone floor. A Rotamix Process Mixing System was the answer.
Extremely flexible in its design, Vaughan Rotamix can be adapted to suit almost any process or tank configuration. The new egg-shaped digester at Changi was no exception. To mix this huge volume of municipal sludge, Rotamix nozzle assemblies had to be strategically placed at different levels inside the digester. Traditional mixing systems, including long-shaft paddle mixers and gas lance systems are not only costly to operate, but can put a great deal of physical strain on a digester roof. With Rotamix, however, nozzle assemblies are permanently installed in place inside the digester with no further adjustment required. Rotamix also has NO moving or rotating parts within the process that could potentially wear out or break – a big advantage when dealing with an application of this scale.
The nozzle arrangement inside Digester 6 consists of four separate nozzle “tiers”, which is made up of a combination of single and dual nozzle assemblies mounted deep inside the cone, at floor level, mid way up the sidewall, and at the very top of the digester. The top nozzles are designed to prevent scum mat formation while at the very bottom of the tank, a set of nozzles keep the cone area well mixed and prevent solids settlement. In addition, two special Foambuster nozzles mounted above the sludge surface will provide protection against surface foaming. Each Rotamix nozzle assembly is constructed from heavy-duty ductile cast iron, which is then epoxy coated and glass-lined and hardened internally for maximum abrasion resistance. For additional peace of mind, each nozzle assembly carries a full 10 year factory warranty against wear and corrosion.
To deliver the required 3,407 m3/hr (15,000 US GPM) of mixing flow to all of the nozzles, two (2) large Vaughan model HE12W18CS mixing chopper pumps were selected. Each pump is direct-coupled to a 90kW high efficiency electric motor to run at a relatively slow speed of 730 RPM. The unique chopping action of the Vaughan Chopper Pump is designed to continuously condition sludge and break down suspended solids. This not only eliminates the risk of nozzle plugging, but ensures that a consistent, high quality sludge blend is always distributed throughout the digester. Rotamix has also been shown to enhance methane production in anaerobic digesters.
Prior to fabrication and supply, detailed CFD simulations were carried out at Vaughan Co. to assess the proposed mixing system’s effectiveness at different sludge concentrations. The CFD study was successful, indicating excellent mixing results.
For further information about this ongoing project or Rotamix in general, please contact us.