Wastewater pre-treatment has stainless at heart Farra Engineering Ltd is a Dunedin-based engineering firm that has been providing engineering solutions for over 140 years. Farra Stainless, a division of Farra Engineering, has built solid reputations in many fields including dairy, food and beverage (including the installation of the award-winning Emerson’s Brewery), architectural construction and lastly but importantly wastewater treatment. The company’s design spirit is illustrated in its innovative rotary screens – a major step forward for the treatment of wastewater in this country.

For all wastewater treatment plants, good mechanical pre-treatment is important for the protection of subsequent processes from clogging, stressing and abrasion. It is a precondition for reliable operation, with little wear and the process, that Farra Stainless has made such a significant impact. Test of time In the mid-1980s Farra Engineering designed and built their first Self-Cleaning Rotary Screens for the Timaru District Council, and in the ensuing 25 years of reliable service they have been proven to be excellent mechanical pre-treatment machines.

The Self-Cleaning Rotary Screens are specially designed for separating solids and liquids, screening, filtration and dewatering. The screens are constructed entirely in 304 or 316 grade stainless steel, depending on the application. In treatment plants, the waste water enters the hydrochamber which has a specifically designed baffle system that reduces the velocity of the incoming wastewater and results in an even flow over the two-sided weir onto the screen. The milliscreen barrel rotates at a constant speed with the cleaned water passing through the wedge wire screen and the solid waste materials being expelled out the end of the barrel for disposal or further processing.

Look Ma, No Hands...
The screens are self-cleaning with internal and external spray pipes with high and low pressure water jets that remove what little physical blockage may occur. This self-cleaning means more capacity, greater efficiency and a reduction in maintenance downtime.

Farra’s screens have proven themselves in the wastewater industry with multiple installations throughout New Zealand. Current projects include the last of four complete refurbishments for Timaru District Council, four new screens for Gisborne District Council’s $21m waste water treatment plant upgrade, one refurbishment of the four screens originally supplied to the Napier City Council in 1990, and the supply of a new screen for the Nelson Regional Council.

The Farra Screens are Not Limited to the Municipal Wastewater Industry
The most common application for the milliscreens is removing suspended solids from industrial and municipal waste water but they can also be used in vegetable processing plants, during grape processing for the wine industry, and for many other trade waste applications. Throughout the years they have been employed in many processes including wool scours, wood fines, paper and pulp, potato and vegetable, tanneries and most recently the wineries. Several new screens/systems are currently being developed to potentially cover all mechanical pretreatment requirements that may be specified by the client or engineer.

Recently, Farra have teamed up with Pumps & Filters Marlborough to produce the hydrawaste complete waste water system. The Hydrawaste system utilizes one of Farra Stainless’ Self Cleaning Rotary Screens to effectively separate liquids and solids. The “hydrawaste” system is excellent for dairy, wineries and industrial processing plants.