Regional Water Recycling Plant No. 2 (RP-2)

Regional Water Recycling Plant No. 2 (RP-2) is located in the City of Chino, and has been in operation since 1960 and treats the biosolids flow streams from the Carbon Canyon Water Recycling and Regional Water Recycling Plant No. 5 (RP-5) facilities. As a result of treating these biosolids, methane gas (or bio-gas) is produced and utilized as a fuel source to operate engine generators that produce electricity. This electricity is used to operate equipment at RP-5 and at the Chino Desalter I facility thereby reducing the Agency’s need to purchase power.

Address
16400 El Prado Road
Chino, California 91708
(909) 993-1765
Fax (909) 393-8129

Plant Description
RP-2 includes treatment processes that concentrate (or thicken), stabilize and dewater the biosolids from RP-5 and the Carbon Canyon Water Recycling Facility (CCWRF). As illustrated in Figure 1, the major treatment process used to thicken the primary solids are Gravity Thickener (GT) Units and for secondary solids, Dissolved Air Flotation Thickener (DAFT) units. After these solids are thickened they are co-mixed and transferred to anaerobic digester Units for stabilization and subsequently, the solids are dewatered and transferred to the Inland Empire Regional Composting Facility (IERCF). In addition, each treatment process is integrated with instrumentation and control systems for controlling and monitoring various aspects of their operations. This overall facility instrumentation and control system is called the Supervisory Control and Data Acquisition (SCADA) System. The following subsections, briefly describe the functionality of each treatment area provided at RP-2.

Biosolids Handling

The Biosolids Handling Facilities at RP-2 consist of three process components including biosolids thickening, treatment (stabilization) and dewatering.

Biosolids Thickening

The biosolids removed from primary treatment and secondary treatment WAS contain a significant amount of water. To further separate the water from the biosolids, two processes form to thicken (or concentrate) the biosolids are provided at RP-2. They include Gravity Thickening (GT) for the primary treatment biosolids and Dissolved Air Floatation Thickening (DAFT) for the secondary treatment WAS biosolids.

Gravity Thickening (GT): The GT process operates similarly to the primary treatment settling tanks only the biosolids are held in the tanks for a longer period of time to concentrate by gravity at the bottom of settling tanks and are subsequently pumped out. Primary treatment biosolids concentrations typically range from about 1 to 3 percent total solids. This means the dry weight or density of a given volume of biosolids mixed with water would be 1 to 3 percent of the total volume. Gravity biosolids thickening can increase the density to about 4 to 5 percent. This makes a significant difference in the amount of water contained in biosolids removed from the GT units.

Dissolved Air Floatation (DAF) Thickening: The DAF process is provided to concentrate the secondary treatment WAS biosolids. Typical WAS biosolids concentrations are about 0.5 percent total solids. DAF thickening can typically increase the density of the biosolids to 4 to 6 percent. This is accomplished by air pressurizing of water so the oxygen dissolves in the water. This pressurized water is mixed with the WAS biosolids as it enters the DAF unit and the pressure is released at the same time. As the pressure is released, the dissolved oxygen comes out of solution in the form of tiny air bubbles.
The WAS biosolids particles collect into to the air bubbles and float to the surface of the tanks (this action is similar to what happens when you shake a carbonated soft drink and remove your thumb from the container opening). As a result, a thick concentrated layer of biosolids is formed on the surface of the tank and skimmed into a hopper for subsequent removal by pumping. To enhance the biosolids collection into the air bubbles and to improve thickening, a coagulant chemical (Polyelectrolyte or Polymer) is used to clump the biosolids particles into larger particles.

Biosolids Treatment

The thickened biosolids from the GT and DAF units are co-mixed and pumped to the biosolids treatment units. These units are referred to as anaerobic digesters which are treatment units that reduce the volume of organic matter by decomposition of the biosolids into relatively stable organic and inorganic compounds from which water will separate more readily. In several ways, anaerobic digesters functions similarly to the human stomach when it digests food. Unlike the activated sludge process, the anaerobic digesters process is carried out in the absence of free oxygen by anaerobic microorganisms. Wastewater biosolids are typically about 70 percent organic and about 30 percent inorganic or mineral. Much of the water in the biosolids is what is called “bound” water which will not readily separate from the biosolids. The anaerobic microorganisms’ breakdown the complex molecular structure of the biosolids which results in releasing the “bound” water. During this release process, the microorganisms obtain oxygen from the water molecule and food from the remaining organic matter.

Time and temperature greatly affects how rapid anaerobic decomposition of the biosolids will occur and the quantity and quality of byproducts such as carbon dioxide, hydrogen sulfide and methane gases produced as a result of the decomposition.

At RP-1, anaerobic digestion occurs in three phases including Acid, thermophilic and mesophilic phases. This allows control on the time and temperature for digestion and stabilization of the biosolids and production of usable methane gas to fuel engine generators that produce electrical power.

Acid Phase Digestion: In this phase of digestion, the microorganisms attack the biosolids soluble and dissolved organic matter such as sugars. From these reactions, organic acids and gases such as carbon dioxide, hydrogen sulfide and low levels of methane are formed. This is known as the acid fermentation phase of anaerobic digestion and this process proceeds rapidly. The operating temperature for this phase is maintained at 100 degrees F.

Thermophilic phase digestion: When completed, the biosolids are transferred from the acid phase digester to thermophilic anaerobic digester units. The thermophilic digesters are operated and maintained at 127 degrees F. This higher temperature provides an environment conducive to a culture of anaerobic microorganisms that will decompose the biosolids organic matter at a higher rate. Therefore, more biosolids can be processed over a shorter period of time. Some acid phase digestion stills occurs in this phase resulting in further release of water from the biosolids. More importantly, this phase rapidly intensifies the decomposition, stabilization and gasification of the biosolids organic matter such as proteins and amino acids resulting in the production and predominance of relatively high quality methane gas. The methane gas is odorless and highly flammable and is used as a fuel source for operating engine generators to produce electrical power.

Mesophilic phase digestion: Upon the biosolids being processed through the thermophilic digestion phase, it is transferred to the mesophilic digester units that are operated at a maintained temperature of 98 degrees F. Digestion of the biosolids organic matter continues in this phase of the digestion process only at a slower rate with additional production of methane gas. At this point, the biosolids are relatively stable and most of the “bound” water has been released to enhance the dewaterability of the biosolids.

Biosolids Dewatering

The relatively stable biosolids are removed from the mesophilic phase digester units on a rotating basis and transferred to the biosolids dewatering facilities. Currently, this facility includes mechanical equipment referred to Belt Filter Presses (BFPs) and centrifuges that are designed to remove water from the biosolids.

BFP biosolids dewatering operation consists of three steps including conditioning, gravity drainage and compression.

Biosolids Conditioning: The biosolids conditioning step is very important and involves the addition of polymers that will help to agglomerate the biosolids particles into flocs and thus provide the initial separation of the solids from the water. This step also conditions the biosolids to build a structure into the solids flocs so they can withstand gradually increasing pressure and shearing action.

Biosolids Gravity Drainage: After conditioning, the flocculated biosolids are deposited onto the gravity drainage section of the BFP. Gravity drainage occurs on a moving belt where the free water created during the conditioning step drains by gravity through minute pores in the belt. This leaves behind a partially dewatered sheet of biosolids. The importance of proper conditioning can be observed at this point. Without proper conditioning, the biosolids will simply pond in the gravity drainage section and run out to the edges of the belt.

Biosolids Compression: As the biosolids travel through the gravity drainage section of the BFP, it enters the compression section of the BFP. This section consist two tensioned porous belts and a series of rollers that sandwich the biosolids to remove additional water. In traveling around the rollers, shear and compression forces are induced to squeeze out water from the biosolids.

The dewatered biosolids characteristic after the compression step is similar to moist mud and is referred to as “Cake”. The cake can typically range from about 16 to 20 percent solids. This means if you had one ton (2000 pounds) of 18 percent biosolids and removed 100 percent of the remaining water, the dry weight of the biosolids would be 360 pounds. Therefore, the higher the cake percent solids, the less water it contains and more solids can be disposed of in the same volume of biosolids.

After the biosolids compression step, it is scraped off the BFP belt and deposited onto a conveyor system. From the conveyor system, it can be stored in hoppers or deposited into trucks and subsequently hauled to the IERCF to be made into compost.

Centrifuge biosolids dewatering operation utilizes centrifugal force to separate the water from the solids. This is typically done in what is called a solid bowl-conveyor sludge centrifuge dewatering assembly.

Biosolids Conditioning: The biosolids conditioning step for the Centrifuge operations is very similar to the BFP operations and involves the addition of polymers that will help to agglomerate the biosolids particles into flocs and thus provide the basis to enhance the separation of the solids from the water through centrifugal force.

Biosolids Cake Formation: The conditioned biosolids slurry enters the high speed rotating bowl through a stationary pipe that extends into a hollow shaft rotating screw conveyor and distributed through ports into a pool. This pool of biosolids takes the form of a concentric annular ring of liquid against the wall of the bowl. As the liquid biosolids flows through the cylindrical section toward the overflow, the finer solids are pushed against the wall while the helical rotating conveyor pushes the solids to the conical section where the solids are forced out of the water and the free water drains from the solids as centrate. The remaining solids or sludge cake is discharged from the Centrifuge by the helical rotating conveyor.

The centrifuge sludge cake is dropped onto a conveyor and combined with the sludge cake from the BFP operation into where it can be stored in hopper or deposited into trucks and subsequently hauled to the IERCF to be made into compost.