Features

July 2008, Vol. 20, No.7

Just Connect

Integrating package control systems with plantwide SCADA systems

justconnect.jpg Penny Larsen and Jim Sizemore

Many municipal treatment plants have a plantwide supervisory control and data acquisition (SCADA) system. Many also have complex package control systems, which typically are provided for equipment supplied with all ancillaries furnished by one vendor, such as centrifuges,ultraviolet disinfection systems, and process air blowers. These package system controls are configured by the manufacturers based on their knowledge and expertise in monitoring and controlling their own equipment. But in order to fully realize the benefits of SCADA, utilities should integrate these package control systems into the larger, centralized system.

The Alexandria (Va.) Sanitation Authority Advanced Wastewater Treatment Facility (ASA AWTF) is a 204,400-m³/d (54-mgd) municipal wastewater treatment plant that recently underwent a full plant upgrade, including the installation of a plantwide SCADA system. The Loudoun Water Broad Run Water Reclamation Facility (BRWRF; Ashburn, Va.) is a new wastewater treatment plant that opened in May. In addition to highly automated SCADA systems, each plant also has a number of package systems, most of which are fully integrated with SCADA. Read full article (login required)

 

Operations Forum Features

Float On and On

Taking flotation thickeners to the limits of performance

floatonandon.jpg John Bratby, Steve Walker, Craig Barnes, and Garr Jones

The Denver Metro Wastewater Reclamation District owns and operates a projected 200-mgd (757,000-m³/d) wastewater treatment plant. In 2020, the plant’s total secondary sludge production is projected to be 144 ton/d (131 Mg/d) dry solids on a maximum monthly basis, with 180 ton/d (163 Mg/d) on a peak-day basis.

The plant uses eight dissolved-air flotation thickener (DAFT) units to thicken secondary sludge, which is then mixed with gravity-thickened primary sludge and digested in anaerobic digesters. Each DAFT unit has an effective surface area of 400 ft² (37 m²) with the following dimensions: 40 x 10 x 12 ft (12 x 3 x 3.7 m).

After 42 years of service, the DAFT units had reached the end of their useful lives. But, Metro recognized the reliability of the DAFT process for thickening waste secondary sludge and its inherent advantages not offered by other modes. Therefore, Metro decided to upgrade and optimize its existing DAFT system to significantly improve long-term reliability and performance.

Some of the principal features of the new DAFT system, including air saturation, entrance structure, and polymer addition, are all important to optimize performance. This article describes the remarkable ability of the new DAFT complex to process extraordinarily high quantities of waste activated sludge (WAS) at loadings up to 235 lb/ft²•d (1150 kg/m²•d) while still providing excellent thickened float concentration and solids capture. Read full article (login required)

 

 Low-Shear Dewatering

 Progress in the search for low-shear, high-solids alternatives

low-sheardewatering.jpg K. Richard Tsang

 

Mechanical dewatering of biosolids is becoming a standard process at many wastewater treatment plants based on the need to transport biosolids long distances for final use and disposal or to subject biosolids to further processing. A 2004 national survey conducted by the North East Biosolids and Residuals Association (NEBRA; Tamworth, N.H.) shows that
more than 900 U.S. plants now have mechanical dewatering, processing more than 1.3 million ton (1.2 Mg) of biosolids per year. Of these, about 65% are dewatered by centrifuges. Centrifuges generally can produce a drier cake but at a higher conditioning chemical dose. Recent studies have shown that the high shear in centrifuges also is a main cause of dewatered-cake odor.

This article explores alternative dewatering devices that do not impose comparable shear on sludge yet provide potentially comparable cake solids. Performance of these technologies, such as rotary and screw presses, can be comparable to centrifuges in some conditions. While these alternatives potentially can perform comparably to high-solids centrifuges, performance data are limited for a wide variety of sludges. Read full article (login required)