Wastewater Collection and Treatment for La Cruz de Huanacaxtle

Lori Wilson/Walt Mintkeski - Amigos de La Cruz
March 21, 2015

Secondary Treatment Aeration Tanks with fine bubble aeration system

Once again, the Amigos de La Cruz would like to thank Walt Mintkeski, professional Environmental Engineer from Portland, Oregon for his informative and educational updates on our local wastewater treatment system. Walt visits La Cruz every year and takes time from his vacation to tour the plant and write his reports so that we can have this valuable information. Thanks Walt, you are truly an Amigo.

The following report was written by Walt Mintkeski. The original report was completed February, 11, 2012 based on two tours of wastewater facilities given on January 25 and February 8, 2012 by a maintenance supervisor of Carcamos (pump stations) and Plantas de Tratamiento de Aguas Residuales (sewage treatment plants) for OROMAPAS in Nayarit. The report was updated on April 22, 2013, based on a tour given February 15, 2013 by the same maintenance supervisor, of the OROMAPAS wastewater treatment plant located approximately 3 kilometers east of Bucerias. It was updated again on March 12, 2015, based on a tour of the new Mega Planta located 3 kilometers east of Mezcales.

Wastewater from the town of La Cruz, the Marina Riviera Nayarit de La Cruz, and a small (.5 liter per second) pump station (on the west side of Arroyo Seco near the entrance to Restaurant Miramar) is collected by a gravity pipe system which conveys it to the Carcamo de Bombeo (Pump Station) de La Cruz. The station is located in the inactive wastewater treatment plant site, which extends between Camaron and Tiburon Streets at the south end of Pez Vela Street. At the southwest corner of the intersection of Tiburon and Pez Vela Streets is the local office of OROMAPAS, which is the government agency responsible for the operation and maintenance of the Carcamo.

The site of the pump station and abandoned treatment plant is most commonly recognized by the concrete channel, which runs through the middle of the plant site, out the south side, across Tiburon Street, and into a larger concrete channel, which empties into the Marina. Water in the channel runs most of the year. During the dry season, the water comes entirely from a spring, which emerges for the ground under a large Huanacaxtle tree on the north side of the plant at the intersection of Pez Vela and Camaron Streets. During the rainy season, stormwater flows into the channel from the east along Camaron Street and from the north along Pez Vela Street.

Before the Marina existed, the plant provided treatment of La Cruz’s wastewater. Initially, a small, extended aeration plant with raw sewage pumps, rectangular aeration tank, rectangular secondary clarifier, and chlorine contact chamber (for disinfection of the treated water) was built on the site on the east side of the concrete channel. As the town’s population grew, a second slightly larger plant was built immediately east of the original plant. It had its own raw sewage pumps, rectangular aeration tank, circular secondary clarifier, and, chlorine contact chamber. In addition, there was a rectangular aerobic digester tank to treat waste activated sludge solids, and a filter press in a small concrete building to dewater the digested solids before they were composted by earthworms in a small rectangular tank. Treated wastewater, disinfected with chlorine, was discharged from the two plants, operating in parallel, via a 10” diameter metal outfall pipe, into the Marina at a point just west of space 12 on Muelle #5 (Dock #5).

The two plants were abandoned at the request of the Marina, which was concerned about their discharge into the marina area. Consequently, the Marina paid for the construction of an 8” diameter pipe (force main), which runs about 3 kilometers from the Carcamo northwest along Tiburon Street, north on Atun, east on the dirt road on the north side of La Joya Condominiums, north on the asphalt road on the west side of Punta Pelicanos to the Punta de Mita Highway, east to Highway 200, and south to the high point on the Puerto Villarta Highway near the Punto Esmeralda Condominiums. La Cruz’s wastewater is pumped from the Carcamo de La Cruz through this pressure pipe and then flows in a gravity pipe to the OROMAPAS pump station in Los Picos. From there sewage is pumped to the OROMAPAS Carcamo #1 in Bucerias, and from there to the existing OROMAPAS wastewater treatment plant east of Bucerias. The plant has a capacity of 60 liters per second (60 lps or 950 gallons per minute or 1.37 million gallons per day).

Approximately 30% of the flow from the Carcamo de La Cruz is diverted from the 8” diameter pressure pipe to an extended aeration treatment plant located in, and owned and operated by, Las Brisas Vallarta Condominiums. The plant has a capacity of 15 liters per second, but normally only treats about 5 to 8 lps. The treated effluent is used for irrigation of the Las Brisas Vallarta property.

When the two treatment plants at the La Cruz site were abandoned, the raw sewage pumps of the larger, newer plant became the Carcamo de Bombeo de La Cruz. During the February 8, 2012 tour, the pump station had 3 submersible pumps (two 25 horsepower and one 35 horsepower) with a total capacity of 18 liters per second (285 gallons per minute or .41 million gallons per day) and producing a pressure of 45 to 50 pounds per square inch (as observed on the pump header pressure gage).

These pumps are located at the end of a long, open concrete channel, approximately 8’ wide by 12’ deep by 30’ long. The channel carries raw sewage from the gravity pipe collection system on Tiburon Street through a bar screen that removes large debris, which could plug the pumps. The channel also acts as a primary clarifier because small rocks, sand, and sewage solids settle out and have to be removed at least once per month by a vacuum (Vactor) truck. Because the sewage solids decompose before being removed by vacuum truck, this open channel is probably the source of occasional odors from the plant site, not the concrete channel, which carries spring water to the Marina.

During the monthly cleaning of the channel, the water level in the channel must be lowered to expose the settled solids so that the vacuum truck can remove them. This would normally be done by turning on more pumps in order to send more sewage through the 8” force main pipe. However, the impellers on the existing pumps could not develop enough pressure to do that. Therefore, a portion of the flow was pumped through the larger of the two abandoned treatment plants to reduce pump pressure and increase flow. This sewage was chlorinated before being discharged through the old 10” outfall pipe into the Marina. Walt Mintkeski witnessed this discharge of partially treated sewage into the Marina on Friday, February 3, 2012 at 9:15 AM while the OROMAPAS vacuum truck was cleaning the channel in the Carcamo.

During the February 8 tour, OROMAPAS stated that its intention was to eliminate this discharge into the Marina during the cleaning process by installing larger capacity sewage pumps to replace the old pumps. However, electrical components had be ordered and installed first before the pumps could be installed.

On February 15, 2013, Walt Mintkeski was given a tour of the existing OROMAPAS wastewater treatment plant, located approximately 3 kilometers east of Bucerias, and a tour of the improvements made at Carcamo de Bombeo (Pump Station) in La Cruz during the past 12 months. The improvements consisted of:

1. Installation of two 23 horsepower (17.2 kilowatt) submersible pumps in place of the three pumps observed a year ago

2. Installation of two new motor control panels with soft starts to slowly start and stop the two pumps

3. Installation of a new emergency diesel generator to provide backup power to the pumps if municipal electricity is lost

The two pumps have sufficient capacity for normal sewage flows, except during heavy rain storms. Because of the improved pumping capacity, it is no longer necessary to divert flow through the old wastewater treatment plant and into the Marina during the monthly channel cleaning operation.

In addition, installation of the emergency generator means that pumping is less likely to be disrupted by problems with the electrical supply system. However the reliability of the pump station has been reduced by that fact that it has only two operational pumps, rather than the three observed 12 months ago.

Engineers Walt Mintkeski and Ibraham Cardenes in front of Secondary Clarifier

Mega Planta

On March 12, 2015, Alfredo Jimenez, with the environment and land development department of the Municipalidad de Bahia de Banderas, arranged a tour of the new Mega Planta for Walt Mintkeski and Duncan Randall (Amigos de LaCruz). The tour was given by Ingeniero Ibraham Cardenes. Located 3 kilometers east of Mezcales, this modern wastewater treatment plant was designed, built, and will be operated and maintained under an 18 year contract with a private company. After the contract expires, the ownership, operation, and maintenance will revert to OROMAPAS. Construction started in November, 2012 and operation began in February, 2015.

The capacity of the plant is 600 liters/second (lps) (13.7 million gallons/day), and it will serve the valley of Banderas Bay from the Rio Ameca on the southeast to the Sierra Vallejo Mountains on the northwest, including La Cruz and Bucerias. However, La Cruz and Bucerias will continue to be served by the OROMAPAS plant east of Bucerias until the second phase of the Mega Planta is completed and a new pump station and pressure pipeline are constructed to convey wastewater from Bucerias to the new plant. At the time of the tour, plant flow was only 100 lps, or 16% of capacity, reflecting the fact that only a few communities have been connected to the plant to date.

The plant is very modern and equals that of plants in the United States. It is designed to remove not only biochemical oxygen demand (BOD: the measure of oxygen needed to degrade the waste in the sewage) and total suspended solids (TSS: the measure of solids suspended in the sewage), but also nitrogen, which is a nutrient responsible for promoting algae growth in rivers where the treated water is discharged. If electric power is lost from CFE, a diesel powered emergency generator will automatically start and power essential equipment.

The treatment process consists of:

Preliminary Treatment, consisting of bar screens to remove trash and rocks; a Parshall flume to measure flow; five 90 horsepower (HP) submersible pumps to lift the flow up to above ground level; a pump wetwell with bubble aeration to remove odors; and an above ground grit and grease removal system;

Secondary Treatment with biological nitrogen removal, consisting of 2 trains of aeration tanks, each with 2 secondary clarifiers. The aeration tanks have an anoxic zone with mixer and pumps to recirculate nitrified mixed liquor for denitrification. The aerobic zone has a plug flow pattern and Sanitare fine bubble diffusers. Air is supplied to the basins by four 200HP Kaeser rotary screw blowers with variable frequency drives which will be controlled by dissolved concentration measured near the basin outlet. The secondary clarifiers separate treated water from the return activated sludge, which is the bacteria solids pumped back to the head of the aeration basin

Disinfection, consisting of Trojan Ultra Violet light equipment (which was not yet installed)

Sludge Digestion, consisting of two centrifuges to thicken waste activated sludge from 2% to 4% solids; one aerobic digester basin per train (with fine bubble diffusers); two larger centrifuges for dewatering digested sludge to 20% solids for trucking to landfill (or agricultural land for application in the future).

At the time of the tour, two secondary treatment trains have been constructed. A third is planned for future growth to meet the full 600 lps capacity.

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