Process Overview

The N-Viro patented process is known as Advanced Alkaline Stabilization with Subsequent Accelerated Drying, or AASSAD.  In this process, an alkaline admixture (AA) is added to dewatered biosolids, mixed, heated (in part through a chemical reaction) and dried. A flow diagram of the typical N-Viro Process and a description of the process unit steps are shown in the N-Viro Process Schematic pictured below:

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Biosolids Sources and Collection

The N-Viro Process can accept a variety of biosolids types including primary, primary/waste activated, waste activated (either digested or undigested) and septage. Mechanically dewatered biosolids typically received at N-Viro facilities will have a solids content between 16 and 35 percent. N-Viro can also accept digested source separated organics.

N-Viro biosolids processing facilities are very flexible in terms of the amount of biosolids that can be processed. They are typically designed to allow increased capacity through longer operating time rather than through a complete retrofit of process equipment. This feature tends to offer a cost reduction to area municipalities who are able to collaborate and together send all of their mechanically dewatered biosolids and septage to a single N-Viro processing facility. As the amount of biosolids processed increases, the overall cost per tonne of biosolids processed decreases.

 
Receiving

The dewatered cake and septage is either transported by truck to a dedicated N-Viro facility or mechanically conveyed to an N-Viro facility integrated within a wastewater treatment plant. In some cases, such as Halifax, specialized trucks are designed to transport biosolids most efficiently.

Biosolids are received in large bins that have augers along the floor. These are known as “Live Bottom Bins” (LBB). The augers convey the material to a discharge which places the material on a conveyor.

 
Processing

Mechanical processing of the biosolids involves two distinct steps: mixing and dryer. Both of these steps contribute greatly to the overall value of the finished product.

Mixing

The biosolids in the receiving area is conveyed to a mixing bin where the alkaline admixture (AA) is added. Typically, 30% - 40% AA is added on a wet weight basis of biosolids. The AA consists of Industrial by-products such as cement kiln dust, lime-kiln dust, and/or fly ash.

The amount of the alkaline admixture varies according to the amount of heating desired in processing, the type of biosolids (the higher the solids content the lower the AA dosage), the characteristics of the alkaline admixture, and the intended beneficial reuse market(s). If the admixture does not contain enough free lime (CaO, Ca (OH) 2 or other strong alkali) to give the necessary temperature and pH rise, CaO is added in the form of quicklime.  The addition of alkaline materials begins the process of pathogen destruction by creating a hostile living environment for micro organisms.

Portable Mixing Capability

N-Viro also offers portable mixing capabilities for clients located in remote locations and for instances where the amount of biosolids to be processed does not warrant the capital investment necessary to build a full-sized N-Viro biosolids processing facility.  The portable mixing equipment may be paired with portable silos, portable dewatering equipment and a portable storage building/shelter.  When portable mixing is employed, the drying step is often omitted.  Photographs of a portable mixer are shown at right:  
 

Drying

The discharge from the mixer travels by conveyor directly into the dryer. The material is now dried to the desirable 60% - 65% solids content with the use of a mechanical rotary-drum dryer. As in the mixing stage, this step is also important in the destruction of harmful pathogens. It is also an important step in the stabilization of heavy metals.

 

Curing

The dryer discharge proceeds to a "heat-pulse cell” where it the material is left to cure for twelve hours. A combination of the heat from the dryer and a chemical reaction between the alkaline materials and the biosolids raises the temperature and pH to a controlled range between 52oC - 62oC and slightly greater than 12, respectively. The reactions taking place also serve to stabilize the product and kill pathogens.

 

Storage

Once the material has cured in the heat pulse cells, it is then transferred to a storage facility for storage and distribution to respective buyers. The material is stable and able to be safely stored. Material is handled with a front-end loader and neatly piled using stacking conveyors. As the material continues to cure to a certain extent while in storage, odours are generated and subsequently treated in accordance with the certificate of approval for air issued to the biosolids processing facility.

 
Air Handling

N-Viro has employed various methods at each facility as a means of preventing fugitive emissions. The facilities in operation to date include air handling systems to ensure the odours and airborne particulates associated with biosolids processing are properly treated and safely discharged back to the environment. There are three main sources of odorous air that require treatment. They include: process building air, dryer exhaust air and storage building air.
 

Process Building Covers and Ventilation

Odours that emanate from the process building are successfully controlled with the use of exhaust hoods, equipment covers and negative building air pressure. The LBB, conveyors, and mixer can easily be designed to operate fully covered with slight negative pressure. This prevents fugitive odours at source.

Dryer Exhaust

The dryer and the dryer cyclone separator are completely enclosed. Air from the dryer flows through the cyclone and is conveyed to a dust removal unit called a baghouse system. The baghouse system is extremely efficient in the removal of airborne particulates in the dryer exhaust. The dust can then be recycled by conveyor back to the process to prevent waste.

Once particulates are removed from the air, further scrubbing is done to remove ammonia and some other odours. Chemical scrubbing and biofilters have been employed either independently or in series with a high degree of success. At one N-Viro facility, chemical scrubbing allows ammonia to be recovered and sold as ammonium sulphate. Ammonia recovery technology is particularly useful as a means of efficiently cleaning process water at locations where water cannot be discharged to the municipal sewer system.

Storage Building Ventilation

The air from the storage building contains ammonia and some airborne particulates that result from product handling. This is normally the largest source of air to be treated at the facility. As such, the storage building air is blended with the dryer exhaust as a means of lowering the inlet air temperature flowing to the scrubbing equipment.