INSTALLATION AND OPERATION OF THE FIRST
LIQUID A™ PROCESS

Tom Hartye, P.E.
Town Engineer/Utilities Director
Hillsborough, North Carolina

Richard W. Christy
President
RDP Company
Norristown, Pennsylvania

INTRODUCTION

The Town of Hillsborough is a small, historic community that traces its roots back to 1754. The Utilities Department has been land applying sludge in a liquid form for many years. Hillsborough recently installed the first Liquid A™ Process in the Spring of 1996. The Liquid A™ Process is designed to operate in a Class A mode or Class B mode.

The Liquid A™ Process is specifically designed to provide a method and apparatus to allow smaller plants to enjoy the benefits associated with producing a Class A quality end product in a liquid state. The Process produces a Class A end product through a combination of heat and lime addition. The operation is conducted on a batch basis in a steel tank that is heated and insulated. Currently, if a smaller plant wants to produce a Class A product, they either have to use an ATAD Process or install a dewatering operation. Both of these options are expensive, often costing over $1,000,000.

In the Class A Process, solids are settled in a clarifier or thickener to concentrate them as much as possible. The settled solids are pumped into a reactor simultaneously with the addition of granular lime. The contents are kept in suspension through the use of an air header while they are being heated. Once the mixture has reached the required pasteurization temperature, the contents are held for the predetermined period of time, as specified in the 503 Regulations. Batch times can be programmed to operate at night and take advantage of lower utility rates.

The Liquid A™ Process overcomes many of the objections to the other options, most notable, the cost. This method of stabilization produces an end product in a liquid form that meets the requirements of Class A or Class B while being in a state that is free-flowing and can be spread by a conventional tanker truck. The end product also contains lime which:

1. Provides the lime that is needed for proper soil pH when biosolids are land applied.
2. The addition of lime makes the end product more marketable.
3. Provides a means of maintaining long-term product stability because regrowth will not occur as long as the pH stays above 9, and lime-stabilized liquid sludges do not tend to see significant pH decay.
4. The use of lime has historically reduced putrefaction odors.
   

PROJECT HISTORY

The Town of Hillsborough has a 3.0 MGD, two stage activated sludge plant which currently runs at around 1.2 MGD and creates approximately 124 dry tons per year at a concentration of approximately

3%. The Town has been land-applying biosolids in liquid form on fields that have been permitted for the past ten years and has recently renewed its permit for 897 acres of mostly fescue pasture land. In 1995, the Town applied biosolids to 294 acres. The current digesters were nearing their capacity to meet the 503 Regulations; and as future growth continues within the service area, they would lose their ability to do so. The Town had also been spending $1,000 to $1,500 per year on lime and was still having problems ascertaining a sludge/soil minimum pH of 6.0 in some of its fields.

CONSIDERATION OF OPTIONS

A Residuals Management Plan was developed to identify and chart the timing of capital improvements that would be needed to stay current with the applicable regulations. The three major components of the Plan are: 1). Permit fields for approximately three times the acreage in use; 2). Install Lime Stabilization to add additional capacity in meeting the 503 Regulations and to meet the sludge/soil pH requirements; 3). Construct a third digester for additional digestion and/or storage. Since Lime Stabilization met two of the Towns required objectives, it was chosen to precede the additional storage component.

Since the Town did not have the room, personnel or capital for considering composting or other processes involving dewatering, the decision was made to upgrade the sludge process to a lime stabilization operation. The cost to lime stabilize the sludge to the Class B level of treatment would have necessitated that the same amount of money be spent to purchase lime for stabilization as is currently being spent to lime the fields. Therefore, the only increase in operating costs associated with this operation would be for the mixing and pumping unit operations associated with the Class B Stabilization Process. These costs were considered to be not significant.

DESCRIPTION OF FACILITIES

Lime Process Equipment

The lime stabilization facilities at Hillsborough consist of a 30 ton bulk lime silo and a reactor vessel that has a nominal capacity of 10,000 gallons. In addition, new slurry pumps were provided along with a blower to agitate and mix the contents of the reactor vessel. The facilities are located on the existing sludge drying beds. One of the beds was converted to a processing area by pouring a concrete foundation for the lime silo and the reactor vessel, thus leaving the remaining drying beds intact. The heart of the system is the reactor tank. The tank is a horizontally-mounted steel tank that is insulated and jacketed. The heat is supplied through electric resistance heating elements. In addition, temperature probes are provided to continuously monitor temperature for both operations and record keeping. The lime is metered in on a preprogrammed basis so that each batch has a controlled and predetermined quantity of lime. The contents of the tank are mixed with an air header in a manner that is similar to an aerated grit chamber. A sample port is provided to allow the pH to be tested after 2 hours and again 22 hours later. When the batch is completed, the contents are pumped to a truck or to a holding tank.

Modifications to Existing Facility

The existing solids handling facility consisted of two 72,000 gallon digesters and 7 drying beds (20 ft. by 40 ft.). The facility was repiped to operate in either of two modes. The first mode provides for normal aerobic digestion using both existing digester in series along with the new lime stabilization equipment operating independently as a batch operation. The tanker trucks can be filled directly from the reactor vessel. This yields one batch of 10,000 gallons (or two "hauls") per 24 hour period over and above what the current aerobic digester yield. The second mode provides for use of the first digester as a thickener/digester and the second digester for lime stabilized product storage. This provides for a continuous operation of lime stabilized sludge production.

CAPITAL COSTS

The cost for the Liquid A™ System was approximately $327,000, the proportional cost for the Liquid B System was $250,000. These costs do not include the contractor’s bid for labor and materials to provide the foundations, install the equipment and connect the electrical devices which amounted to $112,000.

OPERATING COSTS

Class B treatment costs consist of lime and the electricity to operate the pumps and blower. This does not involve an increase in cost over the current conventional process as the cost for the lime is the same (the distribution is significantly easier and involves less labor and coordination), and the difference in electrical costs is minimal.

The direct operating cost for Liquid A™ are primarily the cost of lime and electricity. The following two tables provide a more detailed explanation of these two variables.

DESCRIPTION OF OPERATION

The Liquid A™ Process is a batch operation. The Liquid B Process can be run as a 10,000 gallon batch operation at the reactor vessel or a 70,000 gallon batch at Digester #2. The tank is filled from the underflow of a clarifier or gravity thickener. A predetermined quantity of lime is added simultaneously with the pumping of the biological solids. The electric heating elements are energized for Liquid A™ and the contents of the tank are brought up to the desired temperature and held for a predetermined time in accordance with Alternative 1, Temperature Regimes. The contents are mixed by utilizing an air header.

The lime stabilization facilities in Hillsborough are designed to allow the plant to operate in either a Class A or Class B mode. The system is designed to be fully-automatic. The system control panel incorporates a programmable controller which is designed to control the operation of the pumps that feed the reactor vessel as well as the pumps that discharge from the reactor vessel. In addition, the lime feeder is controlled to add a predetermined quantity of lime to each batch.

The Class B Process has two modes of operation. In the 10,000 gallon batch mode the sludge from either Digester is pumped into the reactor vessel and treated to be hauled out directly after the required 24 hour cycle. In the second mode sludge is pumped out of Digester No.1 into the reactor vessel after a six hour cycle the sludge is pumped to Digester No. 2 and the next of 7 cycles ensues until there is a 70,000 gallon batch in Digester No. 2. After the entire digester has remained for 24 hours at the required pH then it is ready for hauling.

During the course of filling operations, a predetermined amount of lime is added in sufficient quantity to elevate the pH of the biosolids to greater than 12.0. During the operation of the system, an air header is utilized to keep the contents of the tank in suspension as well as to provide uniform mixing.

When the system is operated in a Class A mode, it is necessary to heat the contents of the tank to a pre-determined temperature for a predetermined period of time. Typically, the Hillsborough Plant operates in a manner which requires that the contents of the tank be heated to 150°F and this temperature be maintained for one hour.

The use of a simple programmable logic controller will initiate the operation of the biosolids pump and the operation of the Volumetric Feeder. The tank is filled with the biosolids and then the lime feeder operates for a predetermined time to deliver the predetermined quantity of lime based upon the solids content of the sludge. Testing at the Hillsborough plant has yielded a dosing requirement of approximately 560 lbs. per dry ton of residuals to bring the pH up to 12. An operators reference table located at the main control panel easily relates the required lime dosage set points for various residual concentrations in addition to procedures for adjusting the set point. Throughout the entire cycle the blower is operated to keep the contents of the tank mixed and provide a means of contacting the liquid solids with the hot steel shell. The temperature of the liquid in the tank is sensed by thermocouples and the signals from these thermocouples are used to control the operation of the heating system, much the same as the EnVessel PasteurizationÔ Process. The reactor can be automatically drained at the end of the cycle time or the unloading can be held until the operator has reviewed the time and temperature data and has measured the pH of the contents of the tank.

The end product is ready for immediate land application.

There are several unique features of the lime stabilization facilities at Hillsborough. As far as we know, this is the only lime stabilization system that operates in a totally-enclosed, above-grade steel tank, that incorporates an aeration header to keep the contents of the tank mixed and in suspension. The air is re-circulated from the tank, through the blower and back into the tank's aeration header. The system is totally closed and there are no odors discharged from the reactor vessel. We believe that the re-circulation of the air effectively utilizes the liquid sludge in the tank to scrub the ammonia that is a natural by-product of all lime stabilization operations. This increases the nitrogen content of the finished product by placing ammonia back into a liquid form.

Lime is added in a dry form which reduces the amount of mechanical equipment by eliminating the conventional mix tank, mixer and pump. Additionally, in traditional systems the lime is mixed up in a slurry. This slurry is added to the sewage solids in the lime stabilization process. In the Liquid A™ System, the lime is added in a dry form which reduces the amount of liquid that has to be subsequently applied. It is estimated that this will reduce the volume of liquid biosolids by 10%.

The most unique feature of the Hillsborough System is its ability to operate in either a Class A or a Class B mode. This provides the Town with considerable flexibility in biosolids stabilization operations.

REGULATORY COMPLIANCE

The Liquid A™ Process meets EPA's 503 Regulatory Requirements by utilizing Option 6, pH Adjustment Via Lime Addition to Achieve the Vector Attraction Reduction Requirement. Additionally, the Pathogen Reduction Requirements are accomplished under Alternative 1 which specifies a regime of time and temperature that must be met for solids under 7% concentration.

The pH is adjusted with alkaline materials such as quicklime, hydrated lime, sodium carbonate and other potassium compounds. The pH can be tested after 2 hours and again after an additional 22 hours by drawing a small sample through the sampling port. The temperature is continuously monitored and recorded to document that all of the biosolids comply with the critical time-temperature requirements that are required in order for the end product to qualify as Class A. The reactor vessel is insulated and jacketed to ensure uniform temperature and minimize any potential for heat losses.

END PRODUCT

The uses for the end product from the Liquid A™ Process are unlimited. Turf farms, for example, are an ideal candidate for a liquid Class A product. Quite often, the water content will be greatly welcomed by the turf farmers. In addition, a Class A product allows for more than one harvest per year. This is a major limitation for Class B products because the 503 Regulations limit the number of harvests of turf to once per year. With a Class A product, there are no restrictions on the number of harvests.

Another major use would be pasture land. This application is ideal because the application of biosolids to pasture land is not seasonal and animals do not have to be relocated from the fields and restrict their access for 30 days. Many pastures have not had regular applications of fertilizer or lime; and therefore, they respond very quickly and very well to the application of biosolids. This results in a quick and noticeable improvement in the quantity of growth on the fields and an increase in demand for the biosolids. A Class A designation allows people to try small quantities on a test plot to measure the effectiveness of the biosolids. Considering the condition of most pastures, it is reasonably safe to conclude that most people would be very pleased with the results they obtain from applying the Liquid A™ biosolids product.

Agronomic Analysis of Typical 27,000 Gallon Batch

Producing a Class A Product allows people to try the biosolids and most likely once they have tried it, they will be sold on the end product because it has value. Typically, biosolids in a liquid form, will be applied at a rate of 27,000 gallons per acre. This is equivalent to 1" of rainfall. If the biosolids has 1.5% solids concentration, then it would be reasonable to expect two applications per year. This assumes the solids are 2% to 3% nitrogen and the agronomic loading rates require 150 to 200 pounds of nitrogen per acre. Each application of biosolids would also provide 675 to 1000 pounds of lime. Again, it would be reasonable to expect two applications per year to provide 1350 to 2000 pounds of lime per acre each year. These numbers translate into a reasonable expectation that with Liquid A™, 40 acres for every MGD of flow to the plant would be needed; e.g., a 1.5 MGD plant would only need to be applied twice per year to a single 60 acre site.

CONCLUSIONS AND RECOMMENDATIONS

The Town of Hillsborough's Lime Stabilization Facility has proven to provide the Town with:

1. A low-capital stabilization system.
2. Low operating costs.
3. Effective stabilization with a proven alkaline process.
4. A valuable end product.
5. Operational flexibility to produce a Liquid Class A or Class B product.
   

The experiences gained by the Town of Hillsborough can be used by other small communities who are looking for a practical sludge stabilization operation that is simple, cost-effective and produces a marketable end product.

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