Regional Water Recycling Plant No. 4 (RP-4)

Located in the City of Rancho Cucamonga, the Regional Water Recycling Plant No. 4 (RP-4) has been in operation and producing recycle water since 1997. RP-4 treats an average flow of 5 million gallons per day and is operated in conjunction with RP-1 to provide recycled water to users. The RP-4 facility is being expanded from its current capacity of 7 MGD to 14 MGD.

Address
12811 Sixth Street
Rancho Cucamonga, California 91739
(909) 993-1755
Fax (909) 899-4245

Plant Description
RP-4 includes several treatment processes that contribute to providing a quality recycle water pursuant to the State of California Title 22 regulations. The major treatment processes include raw wastewater pumping, preliminary and primary treatment, primary effluent flow equalization and diversion, secondary treatment and tertiary treatment as illustrated in Figure 1. In addition, each treatment processes 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-4.

Influent Pumping Station
Wastewater from the RP-4 service area collection system flows by gravity into the Influent Pump Station from where it is pumped to the preliminary treatment facility. This station includes multiple pumps that are sequenced to pace the amount of flow entering the treatment plant.

Preliminary and Primary Treatment
Preliminary and primary treatment are physical processes. Preliminary treatment consists of measuring (metering) the quantity of wastewater that flows into the facility, removing large objects and materials with mechanically operated course screens and removing sand and gritty material (inorganic materials that will not decompose). These materials are stored in large bins and subsequently disposed of at a landfill.

Following preliminary treatment, the wastewater is distributed equally to primary treatment settling tanks that allow the wastewater flow to slow down enough to settle out the heavy solids (organic materials that will decompose) by gravity. To enhance settling of these solids, chemicals including ferric chloride and a polyelectrolyte (Polymer) are added to the wastewater. These chemicals help to clump the solids into heavier particles that settle more readily. Typically, primary treatment removes about 65% of the organic solids contained in the wastewater and these solids are referred to as biosolids.

The settled biosolids are continuously removed from the primary settling tanks and transferred to Regional Water Recycle Plant No. 1 (RP-1) where it is concentrated (or thickened), stabilized by anaerobic digestion, dewatered and made into compost for beneficial reuse.

Secondary Treatment
Secondary treatment at RP-4 is a biological process containing millions of microorganisms that can only survive and multiply in an environment containing free oxygen that is dissolved in the water and a food source. This method for treating wastewater is referred to as the activated sludge process. The suspended and dissolve solids in the primary effluent serves as the food source and oxygen is provided by pumping and diffusing air (similar to how air is pumped and diffused in a fish aquarium) into large tanks containing the microorganisms and primary effluent. The mixture of microorganisms, primary effluent and dissolved oxygen is referred to as Mixed Liquor Suspended Solids (MLSS). After the aeration period, the MLSS is transferred to tanks where it is allowed to settle by gravity leaving a clear liquid referred to as secondary effluent. The secondary effluent is then transferred to tertiary treatment while the settled MLSS is returned to the aeration tanks to maintain the appropriate population and cultures of microorganisms.
Return of the MLSS from the secondary clarifier tanks to the front of the aeration tanks is referred to as Return Activated Sludge (RAS). Return of the MLSS from the discharge end of the aeration tanks to the front of the aerations tanks is referred to as ML-RAS. As the microorganisms multiply, the amount of food available in the primary effluent will become insufficient and the quality of treatment can degrade. Therefore, a portion of the MLSS or RAS is wasted from the activated sludge process in order to maintain a balance between the food source and microorganisms that will produce the highest quality of treatment. This wasting is referred to as Waste Activated Sludge (WAS). Like the primary solids, the WAS is also transferred to RP-1 where it is concentrated, stabilized, dewatered and made into compost for beneficial reuse.

Although the activated sludge treatment process produces a high quality of treated wastewater, the State of California, under Title 22 regulations requires the ammonia level in the treated wastewater not exceed a certain limit. Excessive ammonia in the wastewater can have detrimental toxic effects on the environment particularly for aquatic life. Therefore, the biological secondary treatment process at RP-4 is designed to remove nutrients such as ammonia, nitrate and phosphate from the wastewater. This is typically referred to as Bio-Nutrient Removal (BNR).

BNR is accomplished by modifying the activated sludge process so that nitrification and de-nitrification will occur in specific zones within the aeration tanks. These zones are referred to as Oxic (meaning free dissolved oxygen) and Anoxic (meaning devoid of free dissolved oxygen). Nitrification naturally occurs in our rivers, streams and lakes. It is a process whereby microorganisms combined with bio-degradable organic matter, dissolved oxygen in the water and time converts Ammonia-nitrogen to Nitrite-nitrogen to Nitrate-nitrogen which is a more stable organic compound, but can still be detrimental to the environment. Therefore, the activated sludge process is also designed to achieve de-nitrification. De-nitrification is a process similar to the nitrification process only dissolved oxygen in the water is depleted. In the absence of
dissolved oxygen, the microorganisms will utilize the oxygen molecules from the nitrogen compounds for their respiration leaving only the nitrogen that is released to the atmosphere as an inert nitrogen gas.

Tertiary Treatment
Secondary Treatment generally removes the remaining primary effluent suspended and dissolved solids in addition to reducing the level of ammonia, nitrate and phosphate. The solids remaining after secondary treatment typically consists of very fine suspended particles that are readily removed by filtration. Therefore, the nitrified and de-nitrified secondary effluent flows by gravity to tertiary
treatment containing a network of filters designed to remove in excess of 99% of the remaining total solids. The secondary effluent flow is equally distribute to a number of filters containing sand and anthracite coal media that are similar to filters used for swimming pools only on a much larger scale.

As the filters become clogged, they are backwashed one at a time and returned to service. A typical filter run can take 24 to 48 hours before it requires backwashing. The waste backwash water is transferred to a storage pond from where it is pumped into the collection system and sent to the RP-1 facility. To enhance further BNR, ferric chloride is also used prior to filtration for removal of phosphorus. This additional phosphorus removal is required to minimize any impact of the recycled water on the cooling towers at the Reliant Power Plant located across the street from the RP-4 facility.

Reclaimed Wastewater Disinfection
Before the filtered reclaimed wastewater (tertiary effluent and therefore, recycled water) can be use for beneficial reuse, it must be disinfected to comply with the State of California Title 22 bacteriological water quality regulations. This water quality requirement is equal to the bacteriological water quality requirement for potable drinking water as stipulated in the Federal Safe Drinking Water Act regulations.

Disinfection can be somewhat complicated because of the physical, chemical and biological aspects of the recycled water. The disinfecting agent must be well mixed and remain in contact with the recycled water for a minimum time period referred to as Contact Time (CT) to ensure no pathogenic organisms (i.e. disease bearing bacteria and viruses) remain in the water. The unit of measure used to express the CT is milligram-minutes per liter (mg-min/l). The minimum CT requirement is 450 mg-min/l.

The disinfection process is accomplished in chlorine contact tanks where sodium hypochloride, as a liquid chlorine bleach is added to and instantaneously mixed with the recycled water as it enters the tanks. The chlorine contact tanks include baffles to further enhance mixing and contact of the bleach and recycled water. The CT is calculated based on the amount of recycled water being disinfected and samples are taken to measure the bacteriological water quality. The bacterial water quality is measured and express as a Most Probable Number per 100 Milliliters (MPN/100 ml) of sample. The maximum limit is less than 2.2 MPN/100 ml.

Upon being disinfected, the recycled water flows by gravity from the chlorine contact tanks into a common channel and wetwell where it can be discharged to the plant storage pond or pumped to provide utility water within the facility and for beneficial reuse into the recycled water distribution System.

When the demand for utility water or recycled water is less the amount of water being produced, the excess recycled water is discharged to the storage pond and like the filter backwash water is sent to the RP-1 facility.

The disinfection (chlorination) process is automatically controlled to maintain a preset residual. SCADA system instrumentation is provided to measure the recycled water chlorine bleach residual, control the amount of chlorine bleach being pumped for the preset chorine residual setpoint and to monitor and adjust those activities.