Although they may appear crude on the surface, biosolids represent the future of wastewater treatment. This organic substance contains energy and nutrients, and thus the potential for resource recovery and increased energy efficiency by wastewater treatment operations.

But lots of questions around the substance remain. It can be confusing to determine how much treatment is necessary to reuse biosolids and how to achieve it. To get a better handle on biosolids, Water Online spoke with  KLa Systems. We covered  the difference between biosolids classes, a special aerobic digestion process, and how treatment facilities can make the most of their waste.

What do biosolids consist of?
Biosolids are made up of solid and liquid material (also known as sludge)  generated during the treatment of domestic sewage, including solids removed during primary, secondary, or advanced treatment processes.

What challenges do they pose for wastewater treatment operations?
Because the goal of treating sewage is to produce clean water, any of the residual solids removed from the sewage require both further treatment and disposal. The solids treatment processes are capital intensive, and the disposal costs can make up a significant portion of the overall treatment plant’s operating budget. Over the years, both the fertilizer and energy value of these solids has been studied with the goal of generating treatment options to take advantage of the value of the solids while greatly reducing the cost of disposal.

Image credit: “Biosolids 2K8” CityofGeneva  © 2007

What do you make of the potential that biosolids have as renewable sources of fuel and fertilizer?
For treatment plants serving large communities, the generation of biogas from anaerobic sludge digestion is extremely common, and over the past 30 years, the industry has focused on improving the efficiency of the digesters to maximize gas production and cleaning for  optimal  energy recovery,  as  well as reducing biosolids volume through the digestion process and downstream dewatering operations. The use of biosolids  as a fertilizer started in Milwaukee in 1926, when it introduced a pelletized product called Milorganite which was originally sold to golf courses, turf farms,  and nurseries.  Other large cities have made similar biosolids-derived products available to the public. Small and midsize communities have focused more on reducing disposal costs by turning their sludge into a useful soil conditioner.

How can capitalizing on this renewable potential benefit wastewater treatment operations and/ or their communities?Treatment plants that generate biogas where the methane content can be used as an energy source can greatly reduce their costs of treatment. They can also reduce their landfill costs by creating a market for the use of biosolids as a fertilizer or soil conditioner which typically can be available to the local community.

Why are biosolids placed into different class categories?
The categories have to do with the level of pathogens as  well as  the material meeting or exceeding vector attraction reduction  ( VAR) requirements.   VAR refers to processing which makes the biosolids less attractive to vectors (like flies, mosquitoes, rodents, birds, etc.), which have the potential for transmitting diseases directly  to humans.

What do these classes signify?
The  U.S. EPA defines both Class A and Class B biosolids. For Class A biosolids,  pathogens  must be reduced to virtually non-detectable levels, and the biosolids must also comply with strict standards regarding metals, odors, and VAR. Treatment processes that can obtain Class A designation include anaerobic digestion, lime stabilization, composting, autothermal thermophilic aerobic digestion (ATAD), and thermal hydrolysis. This designation means that the material meets  EPA guidelines for land application with no restrictions, meaning the biosolids  can be legally used as fertilizer on farms, vegetable gardens, and can be sold to homeowners as compost or fertilizer.

The term Class A EQ (exceptional quality) is used to describe a biosolids product that not only meets, but exceeds, all Class A pathogen reduction and VAR requirements.  Class  B  biosolids have gone through a lower-level treatment process and can contain higher levels of detectable pathogens than Class A biosolids. The use of Class B biosolids  may require  a permit from the EPA with conditions on land application, crop harvesting, and public access. In terms of nutritional value, Class B and Class A biosolids are similar, as they both contain important nutrients and organic matter.

What is the ATAD process?
The ATAD process is widely used in the U.S.  and Europe  for the stabilization of biosolids in small to  medium-size treatment plants.  It is a thermophilic process operating in a self-sustaining temperature range of 50 to 70 degrees Celsius, without an external heat input, thus providing pathogen inactivation, VAR reduction, and volatile solids reduction sufficient to  meet EPA  guidelines for beneficial reuse. Other advantages of the process include more rapid treatment, a stable process with  minimal  supervision, ability  to accommodate widely varying loads, and a small land footprint.

What are the challenges associated with thermophilic biosolids treatment?
From a sludge-processing perspective, the main challenges are providing sufficient oxygen transfer and mixing to produce and maintain an aerobic environment, control of excessive foaming, nitrogen removal for improved biosolids dewatering, and odor control.

How has KLa’s jet aerator technology overcome those challenges?
Aerating and mixing biosolids is a challenge unto itself. The main reason is  the  total  suspended solids ( TSS) concentration in an ATAD averages 3.5 percent, and this is over 10 times greater than a typical aeration basin. A thicker medium makes mixing and transferring oxygen more challenging.  At KLa Systems, we worked closely with a company  called Thermal Process Systems ( TPS)  that has developed the second generation ATAD. Our joint goal was to develop a jet aeration system that met the needs of their  unique operating environment. Our first step was to create an aeration system operating strategy that could handle thickened biosolids in a harsh, high-temperature environment.

The aerator configuration, pumping, and air delivery design are very different than a conventional jet aeration system in an activated sludge process. The ATAD jet aeration system also required strategies for foam control, varying liquid levels, and maximizing the turn-up/ turn-down of rotating equipment to optimize oxygen transfer performance and maintain optimum energy efficiency. This strategy evolved and was refined over a 5- to 10-year period, and once proven, we had to go back and address some unforeseen issues. The biggest one was related to materials of construction, i.e. excessive jet nozzle wear, due to a combination of the presence of grit with the high-temperature environment, which created a need for a more wear-resistant jet.

This is when we developed the  Kynar  PVDF propulsion jet. There are a number of other technical challenges we have overcome and, in the end, our unique KLa jet aeration system for ATADs has served TPS well, as they now have over 60 successful  ATAD systems producing Class  A biosolids in North America.

What type of plant is ideal for utilizing the ATAD process?
The ideal treatment facility is one that serves a small to midsize community, has modern pretreatment facilities, (particularly grit removal and fine screening), and processes both primary and waste-activated sludge to create Class A biosolids.

 

 

 

 
C/O: WaterOnline