New applications for flotation technology
Bruce Atkinson and Chris Conway of The Environmental Group Ltd explain how a unique method for operating flotation in the mineral industry has been applied with great success to wastewater treatment.
Environmental Group Limited (EGL) acquired the business of Jetflote Pty Ltd in September 1999, including the exclusive worldwide licence of the Jameson Cell technology for water applications. EGL is an Australianowned public environmental technology company listed on the Australian Stock Exchange.
Flotation is an extremely effective means of removing fine particles from liquid wastewater streams. In the process, finely dispersed air bubbles are brought into contact with the chemically conditioned slurry and particlebubble attachment occurs. The particle-laden bubbles float to the surface where they are removed from the cleaned wastewater.
In the Jameson Cell, a low shear method is used to mix the air, untreated wastewater and flocculants. The untreated wastewater and flocculants are introduced into the top of the downcomer at low head. A portion of the clean effluent is recycled back into the top of the down comer. The recycle stream passes through an orifice, accelerating the liquid to produce a simple liquid plunging jet. The kinetic energy of the jet results in air being entrained into the downcomer in much the same way as air might be entrained into a bucket of water using a hose. Air is dragged down into the liquid and the turbulence created around the recycle jet dissipation area breaks up the air film into small bubbles of mean size 300 microns. The Jameson Cell thereby utilises the energy of the fluid to induce air into the cell, rather than requiring an external compressor or blower. The bubble size ranges from below 50 micron up to 1 mm.
The presence of air bubbles at the time of flocculation is extremely beneficial as it results in the bubbles being entrapped within the actual floc structure. The incorporation of bubbles in the floc structure provides buoyancy and allows particles to be floated independent of their surface characteristics.
The downward velocity of the bubble/liquid mixture in the downcomer is chosen such that all bubbles have to descend and emerge into a reservoir (or cell) at the bottom of the downcomer. The reservoir acts as a disengagement zone allowing the aerated floc structures to float to the surface to form a sludge layer. The sludge overflows the reservoir into a launder whilst the cleaned effluent passes to the next stage in the process. Sludge removal is facilitated using either a rotating paddle or chain & flight scraper arrangement.
Advantages over conventional flotation technologies include:
- the cell has minimal moving parts and therefore requires a minimum of maintenance
- no compressor is required
- the process is inherently a low power consumer. The air is self induced as a result of the action of the plunging jet in the vertical down comer. Process power is typically less than 0.1 kWh per cubic metre of water treated
- the low residence time of the cell makes it simple to operate and control
- compact design (total footprint based on rise rate greater than 18 m³ /m² .h)
- the cell is capable of operating at temperatures of up to 70°C as compared to a process relying on dissolved air which is typically ineffective above 45°C due to reduced solubility of the air in the wastewater.
The Jameson Cell technology is widely installed as a primary treatment system for the removal of suspended solids, oil and grease. As a general rule, the IAF process will remove 95-99% of the insoluble components. Flotation cannot remove the soluble components of contaminants unless they can be easily precipitated (eg. phosphorus). The process is particularly suited to removing emulsified fat, oil and grease in addition to the BOD/COD associated with the emulsion and the suspended solids.
A recent application of the Jameson Cell has been for the removal of filter media (eg. diatomaceous earth) from highstrength wine production effluents.
The IAF process has also been found to be extremely effective for biomass separation following secondary treatment. The main advantages are the low footprint and high sludge solids content (4% to 8%) compared to conventional clarification. The process is equally effective for sludges generated from aerobic and anaerobic treatment, and is particularly useful for bulking biomass that typically wont settle in a clarifier.
A major problem with many inland wastewater treatment plants is the proliferation of bluegreen and green algae in wastewater stabilisation ponds. The generation of algae in ponds often results in EPA licence exceedances due to elevated suspended solids and pH levels. Despite this problem, ponds have a number of important advantages which include providing treatment for wet weather bypasses of the secondary treatment process and as an important backup in the event of upstream process failure.
A Research Syndicate was established in 1995 and Jetflote conducted extensive research on developing flotation technology to remove algae from affected wastewater streams. The first stage of the project involved culturing of Microcystis sp. and Anabaena sp. and undertaking laboratory testing to develop appropriate mechanisms for flotation of the algae. The next stage involved construction of an IAF pilot plant to further develop chemical regimes and the optimum configuration for operation of the IAF unit. The pilot plant was extensively evaluated onsite on several algaeladen pond storages.
The proving of this technology was the culmination of three years of research and onsite trialing in the Hunter Valley and other inland sites in Australia where raw water algae levels exceeding one million cells per mL of algae (corresponding to raw water suspended solids levels of the order 150200 mg/L). Treated water algae levels have been generally below 5,000 cells/mL.
Phosphorus: The IAF process can be combined with conventional metal salt precipitation/coagulation of soluble phosphorus is order to simultaneously remove algae and phosphorus from wastewater effluents Very low levels of residual phosphorus can be achieved by singlepoint coagulant dosing of aluminium sulphate or ferric chloride e.g. total phosphorus less than 0.2mg/L can be achieved from raw water orthoP levels in excess of 8 mg/L.
The Jameson Cell IAF technology provides a cost effective alternative to upgrading inland wastewater treatment works since it addresses the problem of algae growth in ponds, thereby allowing water authorities and councils to continue to utilise the benefits of wastewater pond systems. The aim of providing an effective means of simultaneous phosphorus removal is also addressed. The first municipal unit to remove algae and phosphorus from treated sewage (maturation pond) effluent was installed for Wagga Wagga City Council at their Narrung Street Sewage Treatment Plant. The plant, treating up to 18ML/d, was commissioned in June 1999, and has met or exceeded all performance expectations, producing water suitable for discharge to sensitive receiving waters. Process plants have since been installed for Wagga Wagga City Council and North East Region Water Authority.
Jameson Cells are installed in Korea and Malaysia for effluent treatment from compressed timber products manufacture and terminal tank washing facilities. In UK the technology is represented by Brightwater Engineering.
There remain numerous application areas for which the Jameson Cell is likely to be a leading edge technology solution. The IAF process has been demonstrated for several applications in sewage treatment including primary solids removal, clarifier supplementation or replacement, biomass separation and effluent polishing (algae and phosphorus removal). The process is very compact and cost efficient compared to traditional technologies, with a footprint based on a rise rate of 18 m/h or more. The process is well suited to stormwater and combined sewer overflow (CSO) treatment due to the features of low startup time, high process rate, and ease of augmentation to an existing site.
Pilotscale rigs (20 L/min) are available for onsite testwork to confirm process amenability and to evaluate reagent addition requirements.
The article first appeared in the March
edition of WaterJournal of the Australian Water