The plant roots lying immersed in sludge inside concrete enclosures at the city of Princeton’s newly-expanded and modified wastewater plant this fall haven’t looked like much – long tuberous things that might make you think of giant ginger roots.
But these roots that lay there inert as wastewater sludge has been pumped into the enclosures from the city’s wastewater processing stream, are expected to turn into something dramatically different this coming spring. That is, tall green plants that reduce sludge by consuming the sludge’s nutrients, a key link in the wastewater processing at the newly-expanded wastewater facility.
The city of Princeton is at or close to the finish line in its project of not just tripling the city’s wastewater plant’s processing capacity, but also in making some significant modifications in the plant’s design. Another significant design change is in the handling of the wastewater sludge, or biosolids, that are left at the end of processing the wastewater.
In the approximately 16 years since the original mechanical wastewater plant opened, the city has had to remove the sludge from collection tanks and haul it to farm fields about twice a year to spread or inject into the soil.
The wastewater plant’s new design incorporates the use of enclosures called reed beds that can hold biosolids and liquid, and hold a gravel and sand base for reeds to grow in to consume the sludge. The hollow stems of the reeds allow oxygen to come down through the plants to make the process work, says John Fisher, senior design leader at the engineering firm, SEH. SEH is the engineering firm for the city’s wastewater project.
The reed method of breaking down the sludge works so well that no sludge residue should have to be cleaned from the reed beds, likely for 10 years, Fisher said last week.
Fisher notes that reeds have been used for this kind of work throughout Europe, Asia and Australia, as well as in more than 50 locations in the United States. Among its advantages are low construction costs and minimal daily operation and maintenance costs, he points out.
One main advantage is only having to remove sludge residue once every 8-10 years from the plant, versus the old system of twice per year, Fisher said.
The reed system reduces water content, minimizes solids and provides sufficient storage time to stabilize biosolids prior to disposal.
“They’re quite efficient and could grow 8-10 feet tall,” Fisher said of the reeds. “Sometimes it takes one to two years for them to really mature.”
You could compare the reed beds at the city’s wastewater this time of year to that of a marsh that is freezing over for the winter, Fisher added.
Fisher supplied a text on the reeds that describes them as common reed plants with the scientific name, phragmites communis, and a second cousin to the common marsh plant.
The text continues: “It is a tough, adaptable plant, which can grow in polluted waters and find sustenance in sludge. This reed has a voracious appetite for water. The plant is tolerant to low oxygen levels and to waterlogged conditions. The reeds hold themselves in the soil through roots and rhizomes, an intricate network of underground stems.
“New plants in turn will sprout from these stems. These rapidly growing roots provide air passages through the sludge, which in turn provide a host area for many biological communities to develop and continue to mineralize the sludge.”
The Princeton wastewater project resulted in the construction of 12 reed beds, each being in a 50’ x 120’ concrete enclosure. Reeds have only been placed in eight of the reed beds for now because that is all that is considered necessary for now, for processing the amount of wastewater coming through the plant.
The extra reed beds are part of the extra capacity built into the facility to accommodate the potential for future growth.
Fisher noted that the reed beds contain three layers of sand – a pea rock bottom layer, a medium-coarse middle layer and a fine-sand top layer. As Princeton wastewater plant operator Chris Klinghagen looked over some of the reed beds on Dec. 5, he explained that reeds thrive by taking nutrients out of the sludge. He noted that liquid sludge in the reed beds goes into a perforated pipe and the liquid runs to a lift station that pumps it through the wastewater plant again. When the liquid is processed enough through the reed beds, it should exit “crystal clear,” Klinghagen said. “For me, it’s amazing what technology will do.”
Fisher noted that in about 10 years from now, the remaining sludge that will be cleaned out the reed beds will be “pretty inert.” At that point, it can be mixed with compost and applied on land anywhere, according to Fisher, who said it is “quite safe” environmentally.
“It’s nature’s way,” Fisher said about having reeds to reduce biosolids.