Water plant expansion model

The gray structures represent the existing features of the Trinity River Authority surface water treatment plant serving Huntsville. The white structures represent features that would be added during proposed expansion.

The following is a layman's “walk through” of the $99,000 water system capacity study the city of Huntsville hired Austin-based engineering firm Crespo to perform. The report was completed March 29, 2011, before the city of Huntsville imposed voluntary and then mandatory water restrictions this summer in compliance with its drought contingency plan.

The four-page Executive Summary gives the study's conclusions and explains methods Crespo used to determine total system capacity.

The bottom line

• Huntsville needs a water system that can treat and distribute 11.9 million gallons per day.

• The system can reliably treat and distribute 9 to 9.3 million gallons per day or 3 million gallons a day below the required capacity.

These conclusions are based on information collected between 2005-2009 with a period of historic demand of 10.1 million gallons per day set in June 2009. Huntsville set a new record of peak demand over the summer of 11.6 mgd.

Determining capacity

Here’s how Crespo determined how much water Huntsville needs:

“Water supply required is 11.9 mgd based on the equivalents connection analysis.”

In other words:

Crespo used the standard TCEQ formula for determining Huntsville's water demand: the amount of water required to serve a certain number of connections at a certain number of gallons per minute. Coming up with the number of connections to plug into this equation is more complicated in Huntsville because of the presence of the prisons and Sam Houston State University.

There are three ways to do that: 1) count the number of single family homes with city water service; 2)  use the total population according to 2010 Census figures; and 3) use the number of single family homes with water service and add in prison and SHSU water use through a TCEQ formula to come up with “equivalent connections,” also known as “living unit equivalents.”

If you only count single family homes,  24,518 people are served through 8,173 connections, capable of providing 4,904 gallons of water per minute. This population needs a water plant that can treat and distribute 7.1 million gallons per day.

But if you use Method 3, which is what Crespo used, there is a population of 41,223 served by the equivalent of 13,741 connections to deliver some 8,245 gallons of water per minute. This means Huntsville would need a water plant with a capacity of 11.9 million gallons per day.

Crespo trues up this number with the historical monthly maximum usage for the years 2005 to 2009 — 10.1 mgd.

Here's how it determined how much capacity Huntsville's water system has:

Huntsville's water is a mixture of surface water (75 percent) and well water (25 percent). Capacity of the wells is determined by “safe yield” to prevent the unsafe drawdown of the aquifer from which this water is drawn. The production capacity of Huntsville's seven wells is a total of 8.53 million gallons per day — 5.94 mgd from the Palm Street plant and 2.59 mgd from the Spring Lake plant.

But the “total yield” or total capacity is not the same as the rated capacity of the wells to produce “firm yield” or a reliable quantity of water. The firm yield of a well system is its rated capacity. TCEQ requires the city to rate groundwater capacity according to its firm yield, not its safe yield.

The “firm yield” of the wells is the number of million gallons per day that can be pumped from the wells without that largest well in service.

If the largest well goes down, that would have a significant impact on Huntsville's water users. And, as the well field infrastructure ages, the probability of losing the largest pump goes up. Given that the wells are 40 or more years old, Huntsville faces a high probability of losing one of its wells.

The “firm yield” of Huntsville's well field is 5.79 mgd: 4.49 mgd at the Palm Street plant and 1.3 mgd from the Spring Lake plant. The safe yield or the safe pumping rate, from the aquifer, which is 3.3 mgd.

The study also analyzes the firm yield and maximum capacities of the Trinity River Authority surface water plant.

The “firm yield” capacity of the TRA plant is determined by the available water supply from Lake Livingston, the capacity of the plant, and limiting factors that could decrease the amount of water the plant can treat and distribute, such as pump stations, pipes, clarifiers and storage.

TCEQ also requires municipal water treatment plants to rate capacity according to firm yield, not maximum capacity. TCEQ rules require that the component of the plant with the lowest firm capacity be used as the key limiting factor of the plant. In Huntsville's case, the plant component with the lowest firm capacity are the high service pumps, with a firm yield of 6 million gallons per day. Note that the capacity of the plant as a whole is not the same as the capacity of its key limiting factor, the high service pumps.

At the source: Lake Livingston

The firm yield of the plant is also affected by the availability of water at the source — Lake Livingston — either through water levels or Huntsville’s legal right to pull water from the lake.

The total firm yield of Huntsville's water system is the combined firm yield of the wells and the TRA plant.

The minimum capacity required by TCEQ to meet Huntsville's water demand is 2.6 to 2.9 mgd short of  Huntsville's combined rated capacity.

Here's the breakdown:

• Firm capacity of the TRA water treatment plant: 6.0 mgd with a total capacity of 8.0 mgd. The limiting factor: high service pumps, which send treated water into the 30-inch distribution line.

•The short-term safe yield from the aquifer is 3.0 to 3.3 mgd.

•Combined with the groundwater, the surface water system capacity is 9.0 to 9.3 mgd.

• Huntsville is drawing water out of Lake Livingston at a rate close to its permitted limit. If, during a drought,  every water rights holder was pumping from Lake Livingston at their permitted rates, the lake would drop to near empty.

Note that while the models say that, they may not reflect real-world conditions. “This has always seemed alarmist to me,” said Carol Reed, the city's director of public utilities.

Huntsville is among small communities in Walker and surrounding counties with 30 percent of the available water rights on Lake Livingston. The major water rights holder is the city of Houston, with 70 percent.

More for utility geeks and policy makers

Pages 1-13 of the report explain the rigor used to determine number of connections, peak and average water demand, and safe yield of the wells.

The safe yield figure seems to be the one Crespo considers the most variable —  but “conservative but reasonable” — based on modeling and historical data. The data Crespo used is included in the report's two appendices.

Pages 14 and 15 describe the TRA plant and the limiting factors of plant capacity. Voters may want to pay attention to Section 5.2 Potential and Key Limiting Factors. These include drought conditions and availability of “raw” or untreated lake water; lake levels; raw water pump capacity; treatment plant filters and high service pump capacity.

Page 14 notes that the plant's clear well capacity limits capacity to 8 mgd. The clear wells are used to settle solids and sediment from the raw water pulled in from the lake and comprise the first step in treatment.

Page 15 is an evaluation of the capacity of each component of the TRA plant.

The chart on this page shows the firm and total capacity of each component. The number of value to readers is the “firm” capacity or the rated capacity based on TCEQ rules. The total capacity is the design capacity of the individual component. The firm capacity of each component reflects its capacity as part of the system functioning as a whole.

The report identifies the high service pumps as “the key limiting factor” in determining the total capacity of the water treatment plant. If the largest capacity high service pump is down, the system of high service pumps could only manage 6.0 mgd. That 6.0 mgd is considered by the state to be the high service pumps' “firm” or reliable yield.

Note that these high service pumps are 30 years old and atypically enclosed in concrete, which makes them tough to access for servicing, and the parts to the pumps’ electric system are no longer manufactured. Therefore, the probability that one of the pumps will fail is high.

In summary:

• The plant can reliably pull in 16.1 mgd of raw water.

• It can filter 6.72 mgd.  

• It can treat in its clarifiers 8 mgd.

• It can pump 6 mgd.

Page 16 details Huntsville's rights to Lake Livingston water and addresses the impact of drought and the limits of Huntsville's contracted supply to meet future demand.

TRA has the rights to 40,000 acre-feet — about 35.8 mgd — of water per year from Lake Livingston for municipal use. The city of Huntsville entered into a contract with TRA in 1976 for 10 mgd of water per day from Lake Livingston with the option to buy 10 mgd more. In 2006, the city took the option for the additional 10 mgd for a total of 20 mgd. The city took out a separate contract for up to 7 mgd for the Tenaska electric generation plant.

The city has three intake pipes on the Trinity River and Lake Livingston, but only two can be used in an evaluation of Huntsville's intake capacity. The eastern pump is at 112.5 feet and the western pump is at 117.3 feet.

If the level of Lake Livingston drops below 117.5 mean sea level — when the lake is 46 percent full — the western pipe would be out of service. At 32 percent full, or 112.5 msl, the lake would be below the eastern intake pipe.

Lake Livingston is full at 131 feet msl. On Friday, Oct. 21, the lake was at 127 feet msl. The record low was 125.5 msl in October 1988.

On page 17 is a summary of water modeling results to determine likelihood that water levels in the lake would drop low enough to put intake pipes out of service. The report references two software programs known as WRAP, Water Rights Analysis package, and WAM, Water Availability Models, which are commonly used by hydrologists for this purpose.

Note that these models produce theoretical results that might not reflect real-world conditions.

For example, according to WAM, Lake Livingston could drop to near empty in a repeat of the 1950s drought of record. This summer's exceptional drought may qualify as the new drought of record.

WRAP evaluates the effect of climate on Lake Livingston water  levels. The conclusions of a WRAP analysis depend on the beginning assumptions of the modeler. This study used two versions of assumptions, one on current rates of water use and the other assumes water rights holders pull out all the water they're allowed by permit with none of it going back into the lake and river in the form of return flows.

“Current conditions” are an average of hydrologic characteristics of the lake — like rainfall and runoff — from 1940 to 1996. They do not take into account conditions on the lake during the current drought.

Under current conditions, all three of Huntsville's intake pumps on Lake Livingston were functional during the time period included in the model.  But under the second assumption — all water rights used — none of Huntsville's intake pumps would function until “normal” conditions were restored.

Between July 1951, during the drought of record, and July 1996, the water dropped below both intake pipes eight times for as long as 21 months. In August 1955, also during the drought of record, the water level dropped below the elevation of both existing intake pipes for 21 months.

Reed said she questioned Crespo about the modeling results of this chart. The Lake Livingston was not completed until 1969, so the intake lines were not there in 1951 during the drought of record.  The modeling results show what might be true during this period if all water rights were exercised. “Again, my opinion is that this modeling stuff is very confusing and could be misleading,” Reed said.

Page 18 is a summary of results:

The number of connections used to determine demand is 13,741, which results in a minimum required capacity of 11.9 mgd under 2005-2009 conditions.

“This appears to be a reasonably conservative value for a minimum water system design requirement,” the report says.

“Conservative yet reasonable” estimates of well pump capacity show a short-term safe yield from the aquifer of between 3 and 3.3 mgd, according to the report. This is a firm number that can only be changed by drilling a new well with lower pump settings.

Page 19 addresses the effect of critical rainfall shortages on the system operating with one intake pipe out of service — a scenario that could occur during severe drought. In that scenario, even with wells pumping at 50 percent above capacity, the combined water system could only produce 9.9 mgd.

The graph on page 19 shows the firm yield of the city's water system — surface combined with well water — in comparison with average flows over a period from 1995 to 2010. According to this graph, demand for water did not outpace the total firm yield capacity of the system in that time frame.

Page 20 is a list of conclusions, which are stated first in the executive summary.

Pages 21 is a list of references Crespo consulted. Included among them is an Alan Plummer Associates technical analysis from 2005 on the TRA plant and a preliminary report from 2008 on improvements to the plant.

The reports’ appendices contain detailed data on the city's aquifer in part to support its determination of “safe yield” in millions of gallons per day.

Trending Video