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METHODS OF APPLYING BIOSOLIDS
Application of biosolids usually requires special equipment to match the characteristics of the biosolids to the individual site. The amount of moisture in biosolids, commonly reported as % solids (a weight measurement of the amount of solids and water in a biosolids sample), is the predominant characteristic that dictates the type of machinery required, the application procedures, and application timing. The solids content of sludge will vary from a dark liquid at 2-3% solids to a semi-solid moist cake-like material at up to 40% solids. Increasing the solids content of biosolids at the Waste Water Treatment Plant is expensive, and a generator whose use sites are within a reasonable distance will generally be satisfied with a more liquid product. Dewatered biosolids, sometimes called cake, have had polymers or lime added prior to belt filter press or centrifuge processing to achieve a 15-30% solids content. They are generally the consistency of gelatinous mud.
Typical ranges of biosolids solids content applied to restoration sites have included liquid sludge at 2-3% or 6-8% which can easily be pumped, semi-solid biosolids at 8-18% solids which can also be pumped (though less efficiently than liquids), and solid biosolids cake at 20-40% solids which may be flung from a manure-type spreader or end-dumped.
Application rates are typically calculated on a dry weight basis. This means that, for an average dewatered biosolids (20% solids), application of 100 dry t/ac would involve applying 500 wet t/ac of material. This is a significant amount of material - almost 5" deep! This quantity suggests simplicity and speed -- a feature of direct spreading! A variety of equipment technologies are available to perform direct spreading, including farm manure wagons, all terrain vehicles with rear tanks and dump trucks.
Heavy applications such as this can be accomplished using two basic techniques, both of which are relatively easy in concept and cost, but require significant waiting periods for the biosolids to dry out.
- Single application. The fastest and most cost-effective method is to make the total application in a single lift. Depending upon application rate and % solids, this may be as little as 1" to up to 30" in depth! Drying of the biosolids at higher depths may require a complete summer period; drying can be enhanced by seeding with a grass that can germinate and withstand the anaerobic conditions of the biosolids. A cereal grass such as annual rye or wheat is generally very effective for this purpose. Once the biosolids have dried, normal farm disks can be used to incorporate biosolids into the subsoils.
- Multiple lifts. Applications of biosolids can also be made in smaller "lifts," or partial applications. Biosolids are then immediately incorporated into the soil. In fact, in some states, incorporation within a certain time period is a requirement of biosolids management. Incorporation into the soil helps solidify the mixture by dilution of the wet biosolids with the relatively dry soil. However, unless a cover crop is grown before a second application, drying of this mixture may be slower than if the biosolids were simply surface applied. In the case of multiple heavy applications needed within a short period of time, working the soil becomes a definite challenge, as repeated applications followed by mixing without drying will turn the soil into a deep quagmire (potentially far deeper than the actual depth of biosolids added). Because the soil is worked many more times in this method, costs will be significantly higher.
There are several technologies that are effective for applying and even incorporating these rates of materials. Site topography, soil strength, evenness (including debris), and waterways are the physical features that affect equipment selection. Easy access, stable soils and a clear site favor the simple methods, while obstructions or steep slopes require specific equipment. Also important is the application rate, as light applications require a more precise method. The following table summarizes the common types of equipment available to make applications to disturbed soils.
Comparison of different application systems used in remediation sites.
System Range % Solids Relative Costs Advantages Disadvantages Biosolids dump truck discharge, spreading with dozer 10' > 12% Low capital, low O&M Simple to operate, fast for high application rates Need cleared, relatively flat site, acceptable to heavy equipment, difficult to get even applications for low application rates Application vehicle with mounted cannon 125' < 12% Moderate capital, high O&M Can make even applications for low rates, any terrain May need special trails with strength for repeated trips, slow Application vehicle with rear splash plate 10' 15-35% Moderate capital, moderate O&M Can make even applications for low rates, moderate terrain May need special trails with strength for repeated trips, slow Application vehicle with side discharge 200' 15-50% Moderate capital, moderate O&M Can make even applications for low rates, any terrain May need special trails with strength for repeated trips, moderate speed Manure-type spreader with rear discharge 10'-30' > 25% Low capital, low O&M Can make even applications for low rates, moderate terrain Limited to high % solids, trails may need to be close together, moderate speed
Dump Truck and Dozer
The most basic application technologies use dump trucks and bulldozers. Dump trucks can transport materials directly to the application site and end dump accurately without the need for additional equipment for spreading. Biosolids will "flow" out of a truck and spread to a relatively even depth. The trucks need to dump at appropriate distances from each other to assure even application. If the soils can not withstand heavy trucks, either dump trucks or other equipment with high flotation tires can be used between the point that the long-haul vehicles can access and the biosolids are used. This equipment may be available from the POTW that supplies the biosolids, potentially for the price of transportation and a small fee. The capacity of the dump truck combined with the loading or application rate can be used to determine how much ground one load of material should cover. A bulldozer can spread the biosolids over that amount of ground. With the right kind of ground (level to gently sloping) and sufficiently dry soils, this can be a quick and cost effective application technology. The bulldozer will have sufficient traction to drive on ground that has already received application. The process should be staged so that the dump trucks (which will not have sufficient traction) dump at the far end of the site first, then move forward.
Application Vehicle with Cannon
An application system suited to liquid biosolids is a vehicle with a tank and spray nozzle mounted on the rear. Depending on the site needs, a specially designed all-terrain vehicle may be used or a simple heavy-duty truck chassis with rear mounted tank may be acceptable. Each of these systems has been demonstrated to be effective in the Pacific Northwest. The operation of these systems is relatively simple. A biosolids source, where biosolids are transferred into the application vehicle, is available either at the treatment plant, through a delivery truck or from onsite storage. Once full, the vehicle moves onto the site and unloads the biosolids in uniform layers while it is moving or stationary. The vehicle then returns to the biosolids source for a refill and the cycle is repeated. The vehicle-tank spray system is patterned after a combination of fire-fighting systems and log skidders (in the case of the all-terrain vehicle). Key features of the vehicular system include: 1) high ground clearance; 2) suspension that increases tire contact with the ground; 3) articulated steering to reduce vehicle turning radius; and 4) low ground pressure, high flotation, high traction and puncture resistant tires. Key parts of the tank-spray system include: 1) as large a tank as possible, mounted low on the chassis for a low center of gravity to reduce roll-over potential; 2) a pressure-vacuum system for biosolids transfer; and 3) a biosolids or solids pump supplying material to a remotely controlled spray nozzle.
Application Vehicle with Rear Discharge
There are also vehicles that have been specifically designed to apply biosolids to agricultural sites. These typically have flotation tires and a carrying capacity of approximately 18 yards of material. Biosolids are spread from the rear of the box with a fan or splash plate. The width of the spread is comparable to the width of the vehicle. Changing the speed of the vehicle as well as the speed of the fan can alter application rates. These vehicles are excellent for operation on wet soils. The flotation tires generally provide excellent traction and enable access to areas that may not be accessible with conventional equipment. They can spread high or low rates of biosolids or biosolids mixtures onto the surface of a soil. In cases where incorporation is required, additional equipment is needed. Rear discharge application vehicles can also be set up with sub-surface injection equipment which requires a low solids content for proper function. Water can be added to biosolids before application to achieve sufficiently low % solids. The subsurface injection is generally used for agricultural fields that are under a no-till system. It may be appropriate for reclamation projects with relatively low application rates.
Side Cast Spreader
Another type of biosolids application vehicle is a side cast spreader, capable of throw distances of up to 200 ft. Throw distance is dependent on the moisture content of the biosolids, with wetter (15-20% solids) biosolids having a greater throw distance than drier materials such as composts. Application rates can be controlled with this spreader by adjusting the speed of the vehicle as well as the speed of the fan. The spreader can be mounted on a range of vehicles, ranging from a simple truck chassis to an agricultural application vehicle with high floatation tires to all-terrain logging forwarders. The reclamation effort at Palmerton, PA used Aerospreads mounted on surplus army vehicles. This application method is especially useful on very steep or debris-filled sites.
Farm equipment that has been designed for manure spreading also works well for many types of soil reclamation projects. A common design is a wagon pulled by a tractor. Typically, these discharge out the back with a large rotary brush.
Incorporation of high rates of biosolids mixtures similarly require the proper equipment and equipment operators. The low % solids of the biosolids means that when making a 100 dry t/ac application, over 500 wet t/ac of material may actually be applied. Generally a large track bulldozer (such as a Caterpiller D7) pulling a 36" disk is required. Smaller equipment will just float on the surface of the biosolids mixture. Large chisel plows also exist that are capable of incorporating the amendments. When incorporating high rates of amendments it will not be possible to achieve a completely homogenous mixture. Although not always necessary, maximizing soil amendment content will increase the effectiveness of the amendment and should be done where practical.
In most cases, the municipality or private contractor that has applied the biosolids for the municipality will have appropriate application equipment and operators. Arranging for application and incorporation as part of the agreement to use biosolids from a municipality may be the best way to assure appropriate and cost effective application of materials. If the particular municipality does not have appropriate equipment, others will. Examples of municipalities that have large scale application equipment include Chicago (Thomas Granato, 708-222-4063), Virginia (Lee Daniels, firstname.lastname@example.org), Denver (Bob Brobst, email@example.com) and Philidelphia (Bill Toffee, firstname.lastname@example.org). Bob Bastian (EPA Washington, DC, email@example.com) can be contacted as a source of information on application equipment across the country.