July 2008, Vol. 20, No.7

From the Editors

Operations Forum Editor’s Note

Balancing Act

I’m writing this on June 16, and today gas costs just over $4 per gallon, and there is no indication that prices have topped out.

This means higher transportation costs for goods, which leads to higher costs for products and services. Prices are creeping up everywhere, and it’s a constant balancing act to judge the difference between necessities and niceties. And even for the necessities, we should be looking for every way to reduce fuel usage and costs.

Better dewatering for biosolids is one straightforward way to increase the amount of bang for each gasoline buck. Less water in the hauling truck means more solids that get carried to the landfill, farm field, or wherever.

So switching to technologies that produce drier solids is a no-brainer, right? Not exactly.

Making drier solids takes more resources. For example, a belt filter press dewatering a blend of primary and waste activated sludges yields a product that is 23% to 28% solids; a centrifuge yields a product of about 28% to 32%*. But a centrifuge costs more to purchase and requires more polymer and power to operate. So, it’s back to the balancing act to figure out which is best.

Also keep in mind that more vigorous dewatering could also lead to some changes in the product. For instance, higher shear during dewater has been linked to greater odor generation. In some cases, a more odorous product might not be a problem, but in others it could make the product unusable.

Again, we’re back to more balancing.

But now there are alternatives. Some new dewatering techniques promise low shear as well as a high percentage of solids in the final material. The article, “Low-Shear Dewatering ,” explores alternative dewatering devices that potentially could reach the dewatering capabilities of centrifuges, but without the high shear.

Removing more water from solids before digestion enables higher loading, which can lead to more methane cogeneration. That methane then can offset fossil fuels and reduce the demand for them and hopefully slow the increase in fuel costs.

The article, “Float On and On ,” tells how the Denver Metro Wastewater Reclamation District refurbished, upgraded, and optimized its 40-year-old dissolved air flotation thickeners. Through refinement the district has been able to triple the loading rate to the units, as compared to most flotation thickening operations, and still obtain excellent results.

As costs rise, as they inevitably will, these balancing acts will become touchier and more sensitive. The bright side is that ingenuity, imagination, and dedication weigh in on the side of finding more efficient and less costly ways to accomplish the goals at hand.

— Steve Spicer, editor sspicer@wef.org

Percentages taken from WEF’s Operation of Municipal Wastewater Treatment Plants, Manual of Practice 11.