GDDs for Thickening WAS

GRAVITY DRAINAGE DECKS FOR MECHANICAL THICKENING of WASTE ACTIVATED BIOSOLIDS (WAS).

Introduction

The current options for mechanical thickening are Gravity Drainage Decks, Rotating Drum Screen Thickeners (AKA Trommel screens), Rotating Screw Thickeners, Rotating Disc Thickeners, and Centrifuges. All rely on flocculation and filtration.

For efficient, economical and controllable WAS thickening, the Giotto-Magytec Gravity Drainage Deck is the solution! Here’s why-

The Giotto-Magytec Gravity Drainage Decks provide capacities up to 55l/s with thickened solids control and low polymer dosage rates. These features make them the most efficient and economical solution for Waste Activated sludge thickening. They can also be fully enclosed for odour control.

THE OPTIONS

WHAT’S A RST?

Rotary Drum Screen Thickener, (RST), is a drum screen, but Rotating Screw Thickeners also use the RST abbreviation. Big difference. A rotating Drum Screen RST uses continuously cleaned filtration cloth, whereas a fixed screen Rotating Screw Thickener relies on slow scrapers to clean the screen. A clean screen provides a greater filtration rate. That is also why the GDD concept provides the largest unit capacity possible.

PROCESS CONTROL with RST designs

Apart from volumetric capacity limitations, both the RST concepts suffer from limitations of process control. Neither technology offers flexibility to easily control the thickened “cake” solids content. Rotating Screw thickeners particularly can suffer from uncontrolled high cake solids. This might sound like a benefit but can lead to pumping and downstream mixing problems. Some plants even install dilution water systems after thickening to control the thickened solids content. This is obviously wasteful, complex, and expensive.

CENTRIFUGES

Centrifuge thickening of WAS, with high capital cost, and high power and polymer consumption as well as control issues, has led to few installations in recent years.

ODOUR

A perceived benefit of RST technology is that the machines are almost fully enclosed. This helps odour control in applications such as Primary and Digested sludge thickening. Activated sludge, however, is generally innocuous.

Modern Gravity Drainage Deck GDD designs are enclosed, with only the top deck open. This provides the key advantage of visual inspection of the flocculation and drainage process, which is critical for efficient operation. Odour in GDD WAS applications has proven to be negligible in our extensive experience. If still required, full GDD top covers are available.

GRAVITY DRAINAGE DECK

The Gravity Drainage Table/Deck GDD is simply a moving filter belt conveyor. The concept of the Gravity Drainage Table has been used in the paper industry since 1799. The applications for thickening Activated Sludge in Australia date back to at least 1985 when we installed the first Giotto WAS thickening and dewatering cascade at Albury NSW Kremur Street STP. This installation is still in operation after 40 years!

GDD units can offer high unit capacities-typically up to 55l/s in large units. This can reduce the number of required thickening trains by up to a factor of more than 3 when compared to Rotary Screw Thickeners.

Giotto-Magytec GT2000 units, Maroochydore STP Qld.

Then there are the issues of control and polymer consumption. Unlike the RST concept, the Gravity Drainage Deck can be set up to produce a desired thickened solids content. This is important for many downstream applications, in addition to pumping constraints. Centrifuge dewatering can prefer a 4% solids content, and anaerobic digestion typically has a preferred maximum of 6% solids. Both these set points can be readily obtained with a Giotto-Magytec GDD.

With its gentle action, the GDD typically operates with around 50% of the polymer consumption of RST units. Capture rates are typically around 95% compared to 90% for Screw Thickeners.

In summary, if you want efficient, economical and controllable WAS thickening, the Gravity Drainage Deck is the solution!