Ironing out the problems in drinking water treatment plant

Wasserwerk Allmenfeld is a drinking water treatment plant located near Frankfurt, Germany. The plant, owned by Hessenwasser, produces around 36 MLD of potable water per annum. Water is drawn from a number of 40 m deep boreholes and is of good quality before entering the plant with very low turbidity and only trace soluble salts to remove.

Pretreatment comprises aeration of the oxygen-depleted water with air to oxidise soluble iron and increase dissolved oxygen levels. The resultant water then passes green sand biological filters which remove precipitated iron oxides and biologically oxidise permanganate to acceptable levels.

The water is then disinfected before distribution - a very simple and effective process resulting in high-quality water for public consumption.

Apparently little to improve on then; however, historically, the aeration stage had been carried out in large oxidation towers with the air being provided by conventional compressors. In a program of refurbishment, replacement of the oxidation towers and compressors was planned.

The oxidation towers were reaching the end of their usable life and the compressors (each drawing 37 kW) proved expensive to run with reliability issues resulting in frequent outage. Additionally, an oxygen transfer efficiency of around 12% was considered to be a limiting factor in the effectiveness of the downstream biological manganese oxidation stage.

Engineers at Statiflo were asked to provide proposals to replace the ageing redundant equipment with Statiflo gas dispersion systems (GDS).

The system's benefits include: no moving parts (other than a simple pump); low power consumption (less than 15 kW); no maintenance; a simple pipeline installation; no costly pressure vessels; high transfer efficiencies; no compressors to break down; and chemical-free pretreatment.

The systems were supplied with all necessary interconnecting pipework ancillaries and valves required for operation and after a complex retrofit installation (due to the location of existing plant) were successfully commissioned. In service, atmospheric air is drawn into the system by vacuum into the predispersion loop where the gas is sheared into a myriad of tiny bubbles. These micro bubbles present a huge gas/liquid surface area for injection into the main mixer contactors for intimate contact with the bulk water flow and optimum gas dissolving.

The DN 700 main contactor mixers each oxygenate between 500 to 1400 m3/h of water and each feed four filters. There is no requirement for downstream reactor vessels or towers with all gas dissolving being carried out inline.

The performance of the Statiflo GDS units is reported to have exceeded the expectation of Hessenwasser with less than half the power consumption of the original equipment and each system achieves a 67% oxygen transfer efficiency (compared with 12% previously). This efficiency is sustained over a 5:1 turndown in flow rate. Both iron and manganese removal have improved. Water enters the system at a soluble iron concentration of 3.5 mg/L and exits the filtration plant at less than 0.2 mg/L, all in accordance with German drinking water directives.