March 2012, Vol. 24, No.3

The path to net zero

news.jpg For the typical wastewater treatment plant (WWTP), energy costs represent a major outlay. But as technological changes simplify the processes of reducing energy demand and increasing energy generation, more WWTPs may be able to narrow the gap between how much energy they need and how much they produce.

Although significant hurdles to achieving energy neutrality remain, the wastewater field seems poised to begin making strides down the path to net-zero energy status.

Multiple barriers 

For U.S. wastewater agencies looking to implement more-sustainable energy practices, the “No. 1 barrier” is the “unusually low” cost associated with fossil fuels, said Drury Whitlock, U.S. West residuals technology leader at CH2M Hill (Englewood, Colo.). The relatively low cost of fossil fuels results in longer payback periods for capital projects related to renewable energy, Whitlock said.

Economic realities also can thwart efforts to pursue energy neutrality in other ways. Wastewater agencies interested in reducing energy demand or generating energy onsite face “competing demands” for scarce capital funding, said Steve Tarallo, North American business lead for sustainable solutions for the global water business at Black & Veatch (Overland Park, Kan.). Because such funding typically is devoted to projects needed to ensure compliance with discharge requirements and address aging infrastructure, capital projects associated with energy often become lower priorities, Tarallo said.

Meanwhile, wastewater agencies may face another hurdle — their energy utility. In some cases, energy providers have opposed efforts by WWTPs to create their own power, Whitlock noted, either for financial or pragmatic considerations. For example, an energy utility may be concerned about its ability to meet a sudden surge in demand if a WWTP experiences an interruption in its generation of renewable energy.

Although all WWTPs typically can take steps to reduce their energy demand, generating sufficient energy to achieve energy neutrality is a tall order for all but the largest facilities, which enjoy certain advantages, Tarallo said. For example, larger plants typically have lower energy costs on a per-flow-treated basis, and they are more likely to have invested already in anaerobic digestion or incineration, two processes ready-made for energy generation. As a result, attaining energy neutrality may remain out of reach for many facilities.

“Overall, due to the sheer dominance in the number of small plants, I don’t think there will be a significant number of utilities that go all the way to neutrality in the U.S. anytime soon,” Tarallo noted.

 

Steps to take 

Despite the hurdles, WWTPs of all sizes are evaluating ways to move toward energy neutrality. To improve energy performance, smaller wastewater agencies with limited available funds would do well to start slowly and implement small steps, Tarallo said. Reducing energy demand is a good place to begin, because such steps often can be adopted for relatively low capital costs and have fairly brief payback periods.

In the future, WWTPs can expect to have more affordable options available for reducing energy use or generating energy. Among those with great promise is deammonification, the process by which anaerobic ammonia-oxidizing bacteria are used to remove nitrogen from liquid sidestreams. By decreasing aeration requirements, deammonification can reduce a facility’s energy demand significantly. For this reason, the process “has a great deal of potential,” Tarallo said.

 

The promise of codigestion 

For certain WWTPs, codigestion of high-strength organic wastes from outside sources can significantly increase a facility’s power generation capabilities. Facilities best suited to codigestion are those having excess anaerobic digester capacity, located near industrial or institutional sources of carbon-based waste, and with the means to consume or convert the resulting biogas and heat into electricity, said Dennis Dineen, senior wastewater engineer at Donohue & Associates Inc. (Sheboygan, Wis.). By starting slowly with one or two waste sources, a utility can “begin a program without a great investment in capital or operating resources,” Dineen said.

However, such a program requires certain organizational as well as technological capabilities. For example, procedures must be developed for permitting and testing wastes to ensure their compatibility with a facility’s treatment processes. Because successful codigestion programs require a “different kind of business model” compared to that typically followed by wastewater agencies, Dineen said, a utility must determine early on that it is “willing to make the investment” necessary to ensure success. For example, a utility may have to market itself to potential customers and develop alternative forms of billing.

Although codigestion is often the primary means by which WWTPs increase energy production, other opportunities exist, said Rob Ostapczuk, principal engineer at Malcolm Pirnie/ARCADIS (Highlands Ranch, Colo.). Depending on the particular location, microturbines may be used to capture hydraulic energy, heat pumps can capture heat from higher-temperature wastewater, and solar panels can be installed on roofs or other open spaces, Ostapczuk said.

 

Beyond energy-neutral 

For the East Bay Municipal Utility District (EBMUD; Oakland, Calif.), codigestion has enabled the organization to surpass energy neutrality and become a net producer of energy. The district’s 246,000-m3/d (65-mgd) main WWTP can generate roughly 20% more energy than it needs. The achievement is the culmination of years of effort to reduce energy demand and increase energy production at the facility. A decade ago, EBMUD was meeting roughly 50% of its energy demand using internal-combustion engines powered by biogas created during anaerobic digestion. To expand its generating capacity, the district began accepting high-strength organic wastes from nearby industries, increasing biogas production. A new 4.5-MW biogas-powered turbine that came on-line in early 2012 increased EBMUD’s total generating capacity to 11 MW, more than the peak electrical demand experienced by the WWTP during periods of high flows. Beginning Feb. 1, EBMUD officially became a net energy producer, selling 2 MW back to the grid over 2 days.

EBMUD’s ability to generate onsite all the electricity it needs is a “huge benefit” in terms of ensuring facility reliability, said Ed McCormick, EBMUD’s manager of wastewater engineering. Other benefits include revenues associated with the sale of excess electricity to the local electric utility, annual tipping fees, and the prospect of improved air quality once the new turbine comes on-line.

 

Whither energy neutrality? 

By recovering renewable resources and producing valuable products, such as energy, nutrients, and recycled water, wastewater agencies increasingly are in a position to become “green factories,” McCormick said. “At EBMUD, we think 10 years from now, the whole industry is going to be totally transformed,” he said.

However, some question whether most WWTPs can attain energy neutrality without making wholesale changes to standard treatment approaches. “Achieving energy neutrality with conventional aerobic treatment practices is not likely because of the large energy requirement for supplying oxygen,” said Perry McCarty, professor emeritus of civil and environmental engineering at Stanford University (Palo Alto, Calif.). “The only path forward toward energy neutrality that I see is through complete anaerobic treatment of wastewater,” he said. To this end, McCarty is working with researchers at Inha University, in Incheon, South Korea, to develop a novel anaerobic process for treating domestic wastewater.

Because of the expense associated with making major changes to existing treatment systems, energy neutrality may remain out of reach for many WWTPs for years to come, McCarty said. To spur progress, the federal government should sponsor more research efforts and offer financial and regulatory incentives to utilities willing to try new approaches, he said.

— Jay Landers, WE&T 


 

Whose water is it anyway?

Battle for Nevada water rights continues

It has all the makings of a protracted water rights battle.

On one side is the Southern Nevada Water Authority, an agency formed in 1991 to manage the water supply for Las Vegas and the surrounding area. Wishing to reduce the region’s reliance on the Colorado River — which currently supplies 90% of its water — the authority is seeking rights to billions of gallons of groundwater each year from five sparsely populated valleys in eastern and central Nevada. If it succeeds, its plans include constructing a 480-km-long (300-mi-long) multibillion dollar pipeline to transport the water to the Las Vegas area.

On the other side are those opposing the authority’s plan: ranchers, farmers, environmental groups, Native Americans, and others. If the request is granted, they contend, the fragile watersheds and rural lifestyle of the affected region will experience irreparable harm. Opposition groups, such as the Great Basin Water Network (Reno, Nev.), call the project “ill-conceived and underestimated.”

Last fall, both sides had a chance to present their cases to Nevada State Engineer Jason King during a 6-week hearing. As the state’s top water regulator, King now is tasked with reviewing the testimony, wading through tens of thousands of pages of supporting documents, and determining how much groundwater — if any — the authority will be allowed to tap.

King is expected to rule on the authority’s application by the end of March. Then it will be up to the U.S. Bureau of Land Management to determine what the authority can — or can’t — build to pump and transport its allocation across the state.

 

A long time brewing  

This isn’t the first time that water agencies in and around Las Vegas have butted heads with those in other parts of the state.

“[Nevada’s] allotment of the Colorado River water is by far the smallest of any of the seven states that share its flow,” said authority spokesman J.C. Davis.

When the state’s river allocation was set in 1922, Las Vegas had only 2306 residents, compared to a statewide population of 77,000. Clark County, of which Las Vegas is county seat, today houses more than 1.9 million residents, accounting for 75% of the state’s population.

“Renegotiating the original agreement to meet our increased need isn’t feasible,” said Davis. Putting all the region’s eggs in the Colorado River basket isn’t an ideal solution anyway, he said.

The authority already has some existing agreements that are designed to provide a backup water supply, Davis said. But all that water eventually leads back to the Colorado River. “If the river goes dry, these options aren’t viable,” Davis said. “Our goal is to identify a water supply that is independent of the river.”

This, in a nutshell, is what first led the former Las Vegas Valley Water District to apply for rights to groundwater in four rural Nevada valleys in 1989, a request that lay dormant until 2004 — when the region found itself in the midst of a 12-year drought. The former Nevada state engineer’s approval of the water transfers in 2008 was followed by a series of appeals. The Nevada Supreme Court eventually invalidated that decision due to the lengthy time lag between the original application and the decision, and ordered the state engineer to begin the hearing process anew.

 

What’s at stake 

“The fundamental issue the state engineer must address now is whether the Southern Nevada Water Authority’s application meets current water law,” explained Bob Conrad, public information officer for Nevada’s Division of Water Resources.

This means assessing if there is, in fact, water available for appropriation at these sources and, if so, whether the authority’s request conflicts with existing rights, is detrimental to the public interest, or adversely affects domestic wells, Conrad said.

Unlike what might be the case in some other states, the decision is not about whether a major metropolitan area can obtain water from a less-populated part of the state. “The idea of moving water around the state is not novel,” Davis said. “In fact, Nevada water law is very clear. Water is permitted on a first-come, first-served basis. It is not restricted to the aquifer’s actual location.”

Much of the testimony at the hearings last fall, therefore, focused on the environmental and economic impact the authority’s new request would have on the five Nevada valleys whose water it sought. During the 6 weeks of hearings, 82 witnesses testified. Some of the issues they raised demonstrate how difficult these impacts can be to assess.

For example, two of the affected valleys — Spring Valley and Snake Valley — are believed to share groundwater through an interbasin connection. And Snake Valley straddles the Nevada–Utah state line.

Some Utah officials, as a result, worry that pumping water from Spring Valley could potentially cause groundwater levels to decline in Snake Valley, which serves portions of their state.

“It’s very hard to calculate the impact in a quantitative way,” said Hugh Hurlow, a senior scientist at the Utah Geological Survey (Salt Lake City), a nonregulatory division of the Utah Department of Natural Resources. He appeared as an expert witness in the hearings.

“Lower levels in Spring Valley could adversely affect the well levels and springs in southern Snake Valley, which is already under stress due to agriculture and other uses,” Hurlow said.

“Still, if the effect happens, it will take decades to show up in Snake Valley, and even then, the observable change may not be great,” Hurlow added.

Complicating matters further is the fact that no one knows for sure what is going on below the surface. “The connection between the two valleys may be greater than we think,” Hurlow said. “The challenge will be deciding what degree of change is significant and can be attributed to the groundwater pumping. That may be difficult to establish. Other local factors could obscure the signal coming from Spring Valley.”

If water rights are granted, Hurlow advocated installing a monitoring system, including test wells in Utah, that would track changes in groundwater levels and flow for several decades before water transfer begins.

 

Planning for 2050 

“Several decades” of testing may seem like just another delay for a project that already has been debated for more than 20 years. But the authority is not expecting an overnight resolution.

“This is long-term planning,” Davis said. “We’re considering what demand will look like 50 years from now and how to provide the resources needed to supply it.”

In the meantime, the authority is focused on ways to reduce demand through water conservation. As a result of multiple mandatory efforts, annual water usage in Las Vegas dropped by more than 120 million m3 (32 billion gal) between 2002 and 2010, even while the population grew by 420,000, Davis said.

The region currently is using about 75% of its Colorado River allocation. Still, a need for additional supply is coming. “We need a safety net,” Davis said.

No matter what decision the state engineer renders, both sides are fairly certain of what will happen next: more litigation.

“Ultimately, we believe we will be allotted some amount of water from the basins, based on the science available,” Davis said. While the authority doesn’t have a timetable for constructing the pipeline to transport the water, its leaders hope to get all the required approvals and permitting in place so they are ready when the time comes.

Construction of the pipeline, however, appears to be the least of their worries.

“It’s a lot easier to construct a pipe than to get the permission to build it,” Davis said.

— Mary Bufe, WE&T 


 

Bigger trees, better benefits? 

Calculating the returns on your green infrastructure investment

Trees are used frequently as a “green” infrastructure stormwater control in cities. But when it comes to minimizing urban runoff, all trees are not equal.

“[Larger trees] provide a lot more ecological services,” according to Peter MacDonagh, director of design and science at Kestrel Design Group (Minneapolis). For instance, a tree that has a 762-mm (30-in.) diameter-breast-height (DBH), which is the diameter at approximately 1.4 m (4.5 ft) off the ground, provides 70 times the ecological services of a tree with a 76-mm (3-in.) DBH, he said.

In terms of stormwater runoff, Minneapolis uses trees as a part of a green infrastructure strategy to handle the frequent 25-mm-per-24-hr (1 in.-per-24-hr) storms, so gray infrastructure can handle the larger storms, MacDonagh said. During these smaller storms, a 560-mm (22-in.) DBH tree can intercept 80% of the stormwater in its canopy, while a 51-mm (2-in.) DBH tree only can intercept about 15%, he added.

A tree’s size and ability to intercept stormwater often correlate to its age. A 40-year-old tree can intercept 18,925 L/yr (5000 gal/yr) of stormwater, a 20-year-old tree can intercept 5110 L/yr (1350 gal/yr), and a 10-year-old tree can intercept 1890 L/yr (500 gal/yr), MacDonagh said.

However, the average life span of a “street tree” in the 20 largest U.S. cities is only about 13 years, MacDonagh explained. Therefore, the services that urban trees provide often are limited. “We feel that a tree’s life [span] should be at least 50 years,” MacDonagh said.

 

Helping trees thrive 

To live up to their stormwater mitigation potential, trees have to reach a mature age. Planting 100,000 trees so they can grow large is more beneficial than planting a million small trees that will not reach maturity, according to MacDonagh. For example, in Minneapolis, Dutch elm trees larger than 762 mm (30 in.) in DBH constituted 10% of the total tree population but provided about 32% of the stormwater benefits from the city’s tree population, he said.

One of the keys to large, long-lived trees is adequate moisture. If planned properly, trees do not need to be irrigated, because they can rely on stormwater, MacDonagh said. Stormwater can be directed toward trees through pervious paving, catch basins, or trench drains, to provide trees both the moisture and nutrients they need to thrive.

Trees also need an adequate volume of loose, oxygen-rich soil, MacDonagh said. At 85% compaction in soil, tree roots stop growing. “If the soil conditions are better, there’s a much larger range of species that can be put in there,” MacDonagh said. Planting a wide variety of tree species also helps avoid a massive number of tree deaths if a disease affecting a single species spreads through a city, he said.

To avoid compacted soil, structural cells can be installed that support surrounding pavement while maintaining loose soil. With this system, 17 m3 (600 ft3) of shared soil is needed to grow a large tree, while 68 m3 (2400 ft3) of soil would be needed to grow the same tree using structural soil, MacDonagh said.

Excavating even more area, especially horizontally, and then backfilling will have a positive effect on long-term tree growth and is “the least expensive solution,” according to Greg McPherson, research forester with the U.S. Forest Service Pacific Southwest Research Station.

 

Quantifying benefits 

In addition to helping reduce stormwater runoff and intercepting rainfall, trees reduce air pollutants and sequester carbon dioxide. Trees also help keep buildings warm on cold days by blocking wind and cool on hot days by providing shade.

Trees help to reduce the urban heat-island effect because, through evapotranspiration, they disperse heat more efficiently than pavement or masonry, MacDonagh said. During a 29°C (85°F) day, the temperature in the interior of the city would be about 33°C (92°F), but 152 mm (6 in.) above the tree leaves, the temperature will be about 29°C (85°F), he said.

 In the long term, trees with 50-year life spans prove more cost-effective than trees with 13-year life spans, MacDonagh said. It costs $6000 to replace a tree every 13 years for a return on investment of $3000; every $1 invested produces $0.50 in benefits, he said. However, a case study in Minneapolis showed that a group of trees that grew through 50 years without being replaced cost $7500, for a return on investment of $20,000; every $1 invested produced $2.50 to $3 in benefits.

McPherson said the U.S. Forest Service has worked for many years to calculate both the environmental and economic values provided by trees.

“To convert [environmental benefits] into a dollar value is complicated, because no one is paying the tree for doing these things,” McPherson said. The Forest Service equates these benefits to cost savings provided, he noted. For example, it calculates the cost of electricity needed to heat and cool a home during the year without trees and then models the placement of trees around that home to calculate how much the tree saves in energy costs. The agency has developed an online urban tree-benefit calculation tool called i-Tree, available at www.itreetools.org.

          

Planting the right tree 

Even though large trees provide many benefits, not every site in a city can support a large tree. Constraints, such as pipelines or other underground infrastructure, as well as buildings and electrical wires aboveground, “affect the size of the tree that you will plant, because you want a tree that will grow and thrive in the site,” McPherson said. “The goal is to get a tree that will perform admirably for a long period of time with minimum maintenance,” he said.

The optimal size of a tree varies based on site conditions and the goals for the tree function. If a tree is planted for social and aesthetic purposes, it does not have to grow as large as a tree planted to mitigate stormwater or to capture air pollutants, McPherson said.

McPherson also points out that approximately 75% of the land in U.S. cities is privately owned. Therefore, planting the right type of tree with adequate soil and moisture often depends not on professional foresters and arborists but on the public. He encourages citizens to seek out resources and assistance online and through local environmental organizations.

Regardless of constraints, MacDonagh believes that all cities have the potential to increase their total tree canopy. “There’s really no reason in most cities in the United States that [we] can’t obtain a 40% canopy,” he said.

— Jennifer Fulcher, WE&T  


 

A brand new kind of clean

Following a growing trend in the United States, a California wastewater treatment plant adopts liquid chlorine as its new disinfectant

Last year, the San Jose/Santa Clara (Calif.) Water Pollution Control Plant — one of the largest advanced wastewater treatment facilities in California — switched from using chlorine gas to liquid chlorine for disinfection.

“We’ve been using [liquid chlorine] for about a year,” said Jon Newby, deputy director of plant operations and maintenance in San Jose’s Environmental Services Department. In November 2011, the city removed the railroad car where it stored its remaining chlorine gas. “We kept it onsite for redundancy,” Newby said, to make sure the new system with the liquid chlorine worked smoothly and the staff had a chance to adapt.

The 632,000-m3/d (167-mgd) facility is one of many wastewater treatment plants nationwide that has made the transition from chlorine gas to another tertiary treatment method. In March 2010, the Center for American Progress (Washington, D.C.) released a list of 554 drinking water and wastewater treatment plants in 47 states that had switched from “extremely hazardous substances,” such as chlorine gas or sulfur dioxide gas, to liquid chlorine (bleach), ultraviolet disinfection, or calcium hypochlorite.

This change may be motivated partially by proposed legislation. The federal Chemical Facility Anti-Terrorism Standards require all high-risk chemical facilities to complete security vulnerability assessments, develop site security plans, and implement protective measures to meet U.S. Department of Homeland Security risk-based performance standards. Water and wastewater facilities are exempt from these standards, but the U.S. House of Representatives in 2009 passed the Continuing Chemical Facilities Antiterrorism Security Act of 2010 (H.R. 2868), which would have included water utilities under these standards and encouraged hazardous-chemical plants to develop safer and more secure technologies. However, the bill never became law.

Newby said the San Jose/Santa Clara facility was motivated to make the change to liquid chlorine after using chlorine gas for nearly 50 years, because “the requirements for hazardous chemicals were being ratcheted up.”

The City of San Jose had passed a local ordinance regarding hazardous chemicals that required the plant to implement a risk management plan that included training of staff, additional maintenance, and other measures. The plant also had to create an alarm and notification system for the surrounding region in case a chlorine-gas leak occurred. Newby said the Sept. 11 terrorist attacks were another motivation for finding a different disinfection chemical.

“We decided back in the early 2000s to look at different disinfection methods,” Newby said. He said the city conducted feasibility studies of ultraviolet disinfection, liquid chlorine, and advanced oxidation. It found that the lowest-cost, best-known wastewater disinfection method was liquid bleach and sodium bisulfate, he said.

Newby said the transition to the new chemicals and installation of new infrastructure cost the plant approximately $9 million. He said the facility’s chemical cost has doubled, “but that doesn’t mean that the cost of the whole process has doubled.” The cost for total disinfection is about the same when reduced maintenance and administrative costs are factored in, he said. There now is a reduction in manpower needed for training, and the city has to do fewer work orders for maintenance, he said.

— LaShell Stratton-Childers, WE&T 

 

© 2012 Water Environment Federation. All rights reserved.