White Papers

Cost Effectively Meeting California’s Zero Liquid Discharge Requirements

By Mr. Robert Gill, Riverside Public Utilities, Mr. John Baker, P.E., POWER Engineers, Mr. David Tateosian, P.E., POWER Engineers
Presented at PowerGen 2006, November 29, Orlando, FL


Riverside Public Utilities (RPU) is an electric and water municipal utility that serves the City of Riverside, California. RPU recently completed the 96 MW Riverside Energy Resource Center (RERC) which was built to help meet growing needs of this dynamic Southern California city of approximately 285,000 people. RPU’s average system load is about 300 MW with recent peak loads of 587 MW.

During the course of permitting RERC, recently adopted California energy policy was applied to the project which resulted in a significant change to incorporate a zero liquid discharge (ZLD) system to eliminate the discharge of any water from the plant. The addition of the ZLD system represented a significant increase in the project budget whose design evolved to reach the most cost effective solution that would satisfy the plant permitting requirements.

The Riverside Energy Resource Center

The Riverside Energy Resource Center is located on approximately 14 acre site. Development of the project began in May 2003 with commercial operation occurring in May 2006. Reflecting the City’s growing demand and the significant peaking needs of RPU, in the plant’s first forty-five days of operation, the plant was called upon forty-one of those days to generate power.

The RERC is composed of two LM6000 PC SPRINT NxGen combustion turbine generators that produce 96 MW. Turbine inlet air for both units is cooled using a common mechanical chiller that utilizes a wet cooling tower. The plant is equipped with emissions control systems to reduce stack emissions to 2.5 ppm of NOx and is permitted for approximately 2,400 hours of operation per year. RERC uses water for three main purposes:

The RERC is located adjacent to RPU’s Water Quality Control Plant (WQCP). The WQCP receives sanitary waste water from the City and produces treated reclaimed water, a portion of which is sold for landscaping and other appropriate uses. Given the adjacent location of the two facilities, the WQCP was envisioned to be the primary source of plant make-up water and also the receptor for any process waste water.

During the conceptual engineering phase of the RERC, it became clear that the total dissolved solids (TDS) content of the process waste water would exceed what the WQCP could receive. As a result, given the small reduction in TDS that was needed coupled with the peaking application of the plant, the approach of blending some potable water with the waste water stream was selected.

Based on this design, the project proceeded to file its license application with the California Energy Commission under the Small Power Plant Exemption process which could be used for projects under 100 MW.

Permitting Power Plants in California

In California, the California Energy Commission (CEC) has jurisdiction for thermal plants of greater than 50 MW output. Smaller plants are permitted locally through local agencies with jurisdiction.

For plants that are greater than 100 MW in size, the CEC acts as the lead agency under California’s Environmental Quality Act (CEQA) to review the license application. Ultimately once the project is deemed to satisfy all requirements, the CEC issues an Authority For Construction (AFC) after a process that typically takes between 12-18 months to complete.

For plants that are in the range of 50 to 100 MW, the CEC has available to it a simplified process that typically takes between six and nine months to complete. This Small Power Plant Exemption (SPPE) process relies on the CEC still reviewing the project, but exempts it from the full process with local agencies also reviewing the project.

The SPPE process can be used as long as there are no “significant adverse impacts.” For projects under 100 MW where there is a “significant adverse environmental impact,” they must utilize the longer AFC process where such impacts can be processed, and if appropriate, accepted.

Water Use and the Permitting Process

To help guide energy development in California, an Integrated Energy Policy Report (IEPR) has been developed annually for the last few years. The IEPR is a joint effort of the California Energy Commission, California Public Utilities Commission, and the California Power Authority. The IEPR addresses policies for the development of generation and transmission assets for the benefit of the State.

The 2003 IEPR, issued late in the year after the development of the RERC began, addressed policies to mitigate water impacts within State with respect to power plants. The IEPR established a priority of water sources to be used for power plant cooling:

Most Desirable

  • No use or dry cooling to significantly minimize use
  • Zero Liquid Discharge (ZLD) to maximize reuse
  • Brackish, irrigation return, or reclaimed water
  • Well water o Potable Water as a last resort

Least Desirable

As a result, the use of fresh water was intended to be limited to when alternative supplies or technology are either “environmentally undesirable” or “economically unsound.” As an “environmentally undesirable impact” equated to a “significant adverse environmental impact,” this would likely preclude a project considered to have an “environmentally undesirable impact” from utilizing the SPPE process and instead needing to use the longer AFC process.

This aspect is particularly significant for peaking power plants that typically use the SPPE process. The vast majority of combined cycle power plants are greater than 100 MW in size, and thus are subject to the AFC process.

The RERC, as well as two other very similar projects that were also being permitted under an SPPE at the time, became subject to this new policy. As a result, RERC as well as the other plants all adopted ZLD as a part of their design to preserve their use of the SPPE process. This was driven by two considerations:
1. The cost and schedule impact of needing to restart the licensing process as a AFC application with the goal of successfully being able to present a case that ZLD was inappropriate for a peaking power plant, and 

2. The likelihood that ZLD would likely still be required under the longer AFC process.

Application of ZLD at the RERC

With the decision to adopt a ZLD system for RERC, we looked for a permanently installed system to satisfy the following requirements:

1. Receive water from the following sources: 

  • Make-up water from WQCP
  • Cooling tower blow down
  • Oil water separator clean water effluent

2. Produce the following outgoing streams

  • Demineralized water for injection into gas turbines to control NOx and for power augmentation
  • Cooling tower make-up water with acceptable TDS to stay within tower air emissions limits
  • Wet/dry sludge for off-site disposal

Such a system had several advantages as well as some disadvantages:

1. Advantages

  • Established technology recognized by the CEC
  • Reverse osmosis (part of the ZLD system) provided flexibility for inlet water quality
  • Water is reclaimed for re-use reducing overall consumption 

2. Disadvantages:

  • Significant capital cost
  • Higher auxiliary power consumption
  • Additional operator burden
  • Long delivery time
  • Potentially hazardous sludge due to concentration of dilute materials in the reclaimed water

After a significant evaluation it was decided to seek an alternative approach that did not rely on a fully permanent installation due to:

  • Needing to provide a more robust electrical system for an additional 1 MW of load
  • Increasing equipment costs and lead time due to market demand o Impacts to plant infrastructure
  • ~11% increase in project cost which was considered excessive for a 2,400 hour peaking plant

For an alternative, RERC looked to use portable leased demineralizer trailers with installed ancillary equipment to process the incoming reclaimed water as well as the RERC process waste water (largely cooling tower blowdown) to generate demineralized water for plant consumption. With this system, trailers were shipped off site to a nearby regeneration facility. This system still met the ZLD requirement as no waste water leaves the RERC. This approach was still a significant cost, representing an approximate overall 4% increase in the plant’s capital cost. The cost of the demineralized water that is produced is approximately $30-35 per 1000 gallons.

System Operation

Since its installation the system at the RERC, it has proven to be very reliable. Flow of reclaimed water through the system is driven by the supply pressure from the WQCP. This determines flow through the demin trailers. Based on experience to date, each trailer can process approximately 160,000 gallons of process/reclaimed water mixture before needing to be taken off-site for regeneration. Typically a demin trailer is only on-site when it is necessary to make-up to the Demineralized Water Tank. This tank, with a capacity of 300,000 gallons, provides enough capacity for 24 hours of continuous operation of one Unit. Given normal peaking duty, this provides several days worth of supply. Typically a trailer is brought out to begin making more water when the Demineralized Water Tank level falls to 33% full.

Implications for the future

While the State’s implementation of the IEPR policy is not absolute, it does give more reason for power plant developers to look at ways to reduce water consumption. For combined cycle power plants in California, the use of dry cooling and zero liquid discharge systems is almost inescapable.

Similarly, for <100 MW peaking power plants, minimizing water use has become more important to try and avoid the ZLD capital cost and/or AFC permitting process that past water use practices might have engendered. Implications for plant design include reducing water consumption to “de minimus levels” through the use of dry Low NOx combustion turbines and/or using air cooled chillers to reduce water usage for combustion turbine plants. For reciprocating engine based plants with air-cooled radiators, the overall design lends itself to very low water use.

Another alternative to ZLD that is emerging is the use of deep well injection of clean process waste water into aquifers with lower quality water as alternative disposal technology. There are currently at least two projects in California currently making their way through the permitting process that are using this approach.

Thoughts and Conclusions

The primary driver for the IEPR water policy was:

  • California’s growing population
  • Increasing demands on the State’s water resources
  • Increasing expectations for preserving the quality of water

These issues are not unique to California or the Southwest. As similar pressures increase in other areas of the nation, RERC’s experience may provide a useful approach for others too.