Autothermal Thermophilic Aerobic Digestion
(ATAD) System

The thickened waste bio solids are exposed to temperature ranging from 120 degrees to 140 degrees Fahrenheit which disinfects the bio solids. The high temperatures kill pathogens prior to dewatering.

As the majority of untreated wastewater treatment sludges contain solids, organic substances, which are putrescible, as well as pathogenic micro organisms such as bacteria, viruses, worm eggs and plant seeds, a stabilization and sanitation process is desirable prior to ultimate disposal.

The sludge treatment process is AutoThermal Thermophilic aerobic digestion, or simply ATAD. The ATAD process requires insulated reactors, aerators, foam controllers, and sludge.

Reactor 1 and Reactor 2

ATAD System Illustration

Illustration ATAD Reactor 2

However ATAD has been a challenge to operators to control the odor produced from this treatment. The WWTP has installed a bio-filter to help reduce odors from this process.
The air that is being drawn off the ATAD system, first flows through a mister where water and compressed air sprays a mist and removes ammonia form the air.
The bio-filter then removes remaining odors.

ATAD Mister

ATAD Bio-Filter

A continuous pressure-regulated flow of water is supplied to the Liquid Header.
This flow is provided by city water pressure and a Pressure Regulator. The water flows through a filter, before being fed through piping, to the Atomising Spray Nozzle. This Spray Nozzle is located on the side walls of the Reaction Chamber. The odorous air flow enters the top Inlet Duct Nozzle located on the Chamber. The SD-1 Atomising Spray Nozzles, using compressed air as an atomizer, sprays a fog of very fine diluted chemical droplets into the air stream. As the mixture travels through the Reaction Chamber, the water droplets absorb Ammonia and some odor components of the air stream. After absorption occurs in the Chamber, most of the fine droplets collect at the Drain opening in the Reaction Chamber's internal floor and flows into the external Drain piping. The cleansed air stream is exhausted to the mist eliminator to reduce the amount of droplet carryover.

Following the mist eliminator, the scrubber fan draws the air through the scrubber system and then through the bio-filter.
The Compressed Air System provides the necessary air flow to the Spray Nozzle for
atomisation of the liquid chemical stream. In the Compressed Air Line, down-stream
from the Air Receiver Tank, items are installed for control of the compressed air flow.

Bio filters use naturally occurring bacteria to oxidize the odor constituents into harmless by-products like CO2 and water. Most of the time, this treatment can occur with no chemical or nutrient addition. The bacteria can be grown on organic materials like compost, wood chips, tree bark, and in some cases combinations of media. They also can
grow on inorganic surfaces but most of these systems require nutrient addition. Ambio/TPS likes to use coarse ground tree roots (3-6 inches in length) with the fines screened out.
This material lasts 3-7 years and is readily available in most locations. The bio filter at Princeton has a combination of coarse ground root material and pumice. This combination of media has proven to offer a very low back pressure readings on the off gas fan and long media life.

When treating air streams from the first and second generation ATAD systems, several things must be considered to assure success. The air stream is going to have to be cooled so the bacteria can live at ideal temperatures. This temperature range is between 100F and 60F. A water scrubber, using effluent or fresh water usually accomplishes this step. There can be a significant amount of NH3 in the air stream that will have to be reduced before entering the bed.
The scrubber can remove about 80% of the NH3 with just simple water scrubbing. The air needs to be below about 300 PPM NH3 for biological treatment to occur in the bio filter
bed. The air stream from an ATAD is usually close to 100% saturated with moisture, which is very important for treatment in the bed. But, the scrubber step can make sure
that the air is at saturation.

After the scrubbing step, the air is passed through the bed at a proper residence time based on the types and concentration of certain odor constituents. The predominant odor causing constituents in the first generation ATADS are mercaptans and depending on their volumes, the empty bed retention time for the biological step can vary widely. The
actual mercaptan level at Princeton is low (below 15 PPM) in comparison to most other first generation ATADS so the retention time is short, about 45 seconds. The detection
limit that a human can detect mercaptans with his nose is about 50 PPB, so it is very important to let the bacteria in the bed make a large reduction in odor level to minimize
potential for odor complaints off sight. Most of the time, it is possible to reduce the mercaptan level to a point where there is no odor detected other than a wet, musty, woody odor from the media. This usually is not considered offensive and doesn’t travel far from the plant sight. After several years of successful odor treatment, the organic
media will gradually break down and need to be replaced.
This is a slow, predictable process. Simply by using the back pressure on the fan, the plant can tell how much back pressure is occurring each year, and when it is reaching the
maximum fan out put, new media can be located and brought in. Replacing it can be accomplished in a day or two at the most.

Process Performance
On February 19, 1993, the National Sewage Sludge Use and Disposal Regulation (Chapter 40 Code of Federal Regulations Part 503, or commonly referred to as the 503 Regulations) was published in the Federal Register. The regulation addresses several categories of the beneficial reuse and disposal practices for sewage sludge. National standards were implemented in several areas. These standards included:

Heavy Metal concentration limits
Pathogenic limits
Total Hydrocarbon emissions from incinerators
Management of sewage sludge use and disposal

In essence, this ruling established specific criteria in terms of stabilization and pathogenic content for all land applied sludge. It also established different classes of technology which were recognized by the U.S. EPA in meeting these different criteria.

For a sludge to be liquid land applied with no site restrictions, it must meet the following criteria:

Heavy Metal concentration limitations
Vector Attraction Reduction
Time-Temperature Function
Pathogen Testing

Since the ATAD is a biological process, it cannot be used to reduce the heavy metal content of a waste stream. However, the ATAD process will achieve the required vector attraction, time temperature function, and pathogen reduction requirement.

Design Basis
 

Reactor Design
 

Treated Sludge (Post ATAD) Holding Tank

After cooling, the treated sludge is pumped to the Biosolids Building and the Sludge Thickener and Sludge Press as required about once a week.