THIS IS HOW WE DO IT

In the last post it was asked if Dripping Springs and developers there could bust out of their 19th century approach to water resources management. In this post, we look at how they can do that, reviewing a plan for a decentralized concept wastewater system, with the reclaimed water used for irrigation. And in the next post we’ll look at a stormwater management plan based on distributed Low-Impact Development (LID) techniques, integrated with rainwater harvesting to also supply irrigation water. This integrated water management strategy is a 21st century approach.

To address our 21st century water challenges, the over-arching goal of employing these strategies is to pretty much take irrigation off the potable water system. It is typically expected that, averaged over the year, about 40% of projected water demands would be for irrigation. So if we attain our aim, the actual water required for new development could be only 60% of the currently projected demand. That would drastically reduce the strain on existing supplies from local groundwater and the Highland Lakes, and it would blunt the need to develop new supplies, like schemes to import water to this area from the Simsboro Aquifer. That would be very expensive and likely unsustainable over the long term, as reviewed in “One More Generation”. Decentralized concept wastewater systems and LID stormwater management can therefore help to move us toward sustainable water in this region.

A neighborhood in the proposed Headwaters project is used to offer an example of these techniques. This is one of the developments in the hinterlands around Dripping Springs to which the city is proposing to extend an interceptor and take their wastewater “away”. An overall draft plan for Headwaters is shown below. The neighborhood we’ll focus on is along the first side street you come to as you run on down the main road entering the project off U.S. Hwy. 290. This street and one cul-de-sac off of it are fronted by 29 houses.

hd conceptual yield study - full plan 2014

[click on image to enlarge]

 

DECENTRALIZED CONCEPT WASTEWATER SYSTEM

Before proceeding to review the details within this neighborhood, let’s look at how the decentralized concept strategy works with the “time value of money”. The development-scale conventional centralized wastewater system currently permitted for this project would have the treatment plant located in the lowest area down toward Barton Creek. So if the development were to begin with lots on the “higher” end, closer to Hwy. 290 to minimize the length of the main road and waterline to be initially installed, then they’d have to build a long run of wastewater interceptor main down to the treatment plant site, all of which would have to be sized to carry the flow from all of the development that would eventually occur along its route. This would impart a large “carrying charge” since much of that development wouldn’t be built for many years, thus much of that investment would lie “idle” for a long time. If, on the other hand, the first neighborhoods to be developed were down close to the treatment plant site, then a long run of road and waterline would have to be built, also imparting a “carrying charge”. As we will see, under the decentralized concept strategy, the whole wastewater system for each neighborhood is self-contained, built on a “just in time” basis, so one could start development anywhere desired without incurring “carrying charges” for that function.

Back to that example neighborhood, a plan approximating its layout is shown below, along with a decentralized concept wastewater system to serve those 29 houses. The plan utilizes the three essential tools of the decentralized concept to simplify the system, to make it more robust, to reduce its costs, and to maximize water utilization efficiency:

  • Effluent sewerage.
  • “Fail-safe” treatment.
  • Beneficial reuse of the water resource.

Headwaters neighborhood ww sketch plan

[click on image to enlarge]

Effluent Sewers – Simpler and Less Costly

In an effluent sewer system, wastewater from a house runs through the house drain into an interceptor tank (primary septic tank). These interceptor tanks hold and digest the settleable solids, so that only a liquefied effluent would run to the treatment unit. This allows use of small-diameter effluent sewer lines. These can run at very small and variable grades, typically with the lay of the land, in shallow, narrow trenches.

The interested reader can find a thorough review of the effluent sewer concept and its advantages here. But basically we use it because it is less expensive than conventional large-pipe sewers that have to run on larger and uniform grades, and it creates a simple, easy sludge management system – just pumping the interceptor tanks at multi-year intervals. Because the whole system is contained within the neighborhood, the overall cost of the collection system would be significantly less than a conventional collection system. That system would include not only the more costly lines within this neighborhood but also the large interceptor mains outside the neighborhood leading to the centralized treatment plant, the lines that would incur those “carrying charges”.

Ideally the area tributary to a treatment unit would be those houses that could drain by gravity through an effluent sewer system to the plant location. But where the topography dictates, one or more interceptor tanks could drain to a pump station, as shown in the drawing, to be pumped from there to the treatment unit, or to a point in the pipe system where gravity flow could take over. Such pump lines would also be small-diameter pipes installed in shallow, narrow trenches. Where they run parallel to the effluent gravity sewer, they would be in the same trench, so the cost of the pressure sewer would essentially be just the cost of the second pipe in that trench. Also an effluent pump station would be simpler and less problematic, and significantly less costly, than a conventional lift station, which would be needed at that point in a conventional centralized system.

“Fail-Safe” Treatment is Essential

The concept of “fail-safe” treatment bears a bit of explanation. I always use quotes in setting forth this tool, as nothing is ever completely fail-safe. Every sort of treatment unit will need proper operations and maintenance (O&M) in order to continue to function over time. However, there are some treatment technologies which, by their very nature, are resilient and robust, whose failure modes are slow and gradual, so can consistently and reliably produce a high quality effluent even in the face of temporarily poor operating conditions. One such technology is a variant of recirculating sand filter technology that I have labeled the high performance biofiltration concept. The interested reader can go here to get a thorough rundown of how this concept works and why it is highly robust, able to run with minimal routine oversight.

That’s a sharp contrast to the inherently unstable activated sludge process that is almost exclusively used by the mainstream in their centralized plants. And it is essential to a strategy entailing many small, distributed treatment units. Using activated sludge plants as distributed treatment units would be a disaster, as the O&M liabilities would be untenable. The high performance biofiltration concept, however, can run with little active oversight over long periods, so policing multiple plant sites would not create a great burden. This simple operating concept also uses far less energy than an activated sludge plant.

This treatment system will consistently and reliably produce an effluent quality better than that produced by most municipal treatment plants, including removing well more than half of the nitrogen from the wastewater. Recall from the last post that nitrogen was identified as a problematic pollutant, so care must be taken to remove it before any of the reclaimed water might seep into a creek.

Maximizing Irrigation Reuse, Minimizing Pollution

The reclaimed water coming out of the treatment unit would be routed into subsurface drip irrigation fields, arrayed as much as possible to irrigate areas that would be irrigated in any case, so maximizing the reuse value of this water resource. As the plan shows, much of the reclaimed water feed pipe could run in a common trench with the effluent sewer pipes. So the cost of much of this distribution system would basically be just the cost of the second pipe in that trench.

As noted, this decentralized concept plan aims to take irrigation demands off the potable water system. The plan shows dispersal of the reclaimed water is focused on front and side yards and parkways, in the “public” spaces, leaving the back yards – the “private” spaces – unencumbered by the drip fields, allowing the owners to install patios, pools, etc., there. (As will be reviewed in the next post, those private areas could be irrigated with harvested rainwater, integrating that into the stormwater management scheme, so taking that irrigation off the potable water system as well.)

The area of drip irrigation field is based on a design hydraulic application rate onto the drip fields of 0.1 gallon per square foot per day, about the average year-round evapotranspiration rate in this area. This plan provides more than enough space in areas that could be beneficially irrigated to meet that criterion. Note however this rate dictates that, on average, the field would be under-loaded through the heat of the summer and over-loaded through the winter. Thus, the drip field would act like a “drainfield” through part of the year, and indeed on any days at any time of year when a significant amount of rain fell. This may raise a concern about public health and environmental protection.

With the high quality effluent produced by the high performance biofiltration concept, and adding on UV (ultraviolet) disinfection as a safety factor, subsurface dispersal would pose no public health hazard. The water would be sequestered below the surface so contact potential would be extremely low. And any water that percolates through the soil, perhaps eventually to emerge at seeps as is likely in this topography, would have been completely “renovated” by passage through the improved soil that would be installed over the drip field areas.

Regarding environmental protection, the nitrogen concentration of the wastewater will be knocked down in the treatment unit, so it would be loaded at a rate more closely matching the uptake of nitrogen by plants covering the drip fields. Over the annual cycle the vast majority of the reclaimed water entering the drip irrigation fields would exit by way of evapotranspiration into the air instead of percolation down into the soil, where it could perhaps migrate to seeps and on into streams. So the potential mass loading of nitrogen into streams would be inherently very low. In any case, it can be expected that most of the nitrogen in the reclaimed water that is not taken up by plants would be eliminated by in-soil denitrification, gassing off the nitrogen into the atmosphere in the same manner it is eliminated in the treatment process. The soil is also by far the best medium for eliminating/assimilating the contaminants of emerging concern, such as pharmaceuticals, which would be so very problematic if the effluent were discharged to a stream.

Indeed, critics of dispersing the reclaimed water in uncontrolled access areas as shown in the plan – front and side yards, parkways, a park – would be hard pressed to show why this would be unsound practice in regard to either public health or environmental protection. As reviewed in “Slashing pollution, saving water – the classic win-win (but ignored by society)”, our controlling institutions allow – indeed they support – the spewing of water over the surface that has been questionably treated in home-sized activated sludge units subjected to a meaningless level of oversight, and then is rather questionably disinfected in a drop-feed tablet chlorinator, all over Hill Country watersheds. And many other houses have subsurface drainfields dispersing water into the soil, with no organized oversight at all. Contrast that with what is posed here – a professionally managed, highly robust and resilient treatment unit with subsurface dispersal at irrigation rates after highly effective UV disinfection – inherently far less problematic.

By the same token, concerns about marketability of a development with this sort of wastewater management system ring hollow. Again, there are all those houses being sold with that (smelly) activated sludge unit sitting right next to the house, with the poorly treated water sprayed around the lot. Whole large subdivisions, including some within Dripping Springs’ jurisdiction, employ that as the wastewater management strategy, and the builders don’t seem to be batting an eye. Indeed, it is the builders who insist upon installing the relatively cheap activated sludge and spray dispersal system, who refuse to consider using a “fail-safe” treatment unit and drip irrigation. So to suggest that the decentralized concept scheme would negatively impact on marketability is disingenuous, to say the least.

Back to real issues, irrigation of front and side yards with the reclaimed water would relieve the homeowners of water bills to irrigate these spaces. But, as noted, the amount of water each house would produce as wastewater would leave these areas under-loaded through the peak irrigation season, IF a conventional turf and/or high water demanding “exotic” plants were used to create those front yard landscapes. This suggests another strategy to match the needs of the landscape to the water made available through the wastewater system – a regionally appropriate landscaping aesthetic/ethic. A front yard landscape employing native and native-adapted plants, such as shown in the picture below, could thrive on the amount of water the wastewater system could provide, so no draw on the potable water system for additional irrigation would be needed through the peak irrigation season.

OLYMPUS DIGITAL CAMERA

[click on image to enlarge]

This sort of landscape might be institutionalized as the “face” of this development, displacing the sterile patch of water-guzzling turf that is the “stock” aesthetic in such places. The developer might deliver the home to the buyer with a basic native plant palette in place over the mulched and improved soil bed, as required to support drip dispersal of reclaimed water. The homeowner might be given an account at a participating native plant nursery and some assistance/instruction in native plants so that he/she can enhance the landscape as desired, creating buy-in to this aesthetic.

What Does It Cost?

Rough cost estimates were made for the effluent sewer system, the treatment unit, and the reclaimed water feed system shown in this neighborhood plan, yielding an estimate of about $8,000/house. To complete the entire system, the cost of the drip irrigation fields would have to be added. It is called to question however if those are not, in part at least, costs that would be borne in any case, given that much of the area shown on the plan as irrigation field might be irrigated anyway. It can be argued that, in this terrain, amended soil would cover the front yards and parkways to support improved landscaping without regard to whether it would be needed to support environmentally sound drip dispersal of the reclaimed water. Indeed, minimum soil depth on lots for landscaping is required under the Fish & Wildlife MOU that would allow this development to use water delivered by the Hwy. 290 pipeline from Lake Travis. And installing drip lines in this amended soil would not be significantly more costly than a spray irrigation system, which the drip fields displace.

While hard to compare without more details than I currently have, it is expected that these costs compare well with what would be needed to implement the conventional centralized system. The conventional collection system within this neighborhood would incur a similar cost to the effluent sewer system within it, with the interceptor tanks included, and then to that you’d have to add a share of the cost of the interceptors and lift stations needed to get wastewater from this neighborhood down to the centralized treatment plant. That plant would no doubt cost less per gallon per day of capacity than the small decentralized plant, but here again the centralized plant would be sized for the flow at buildout. So the total cost would be much greater, with the capacity that would not be fully utilized for many years imparting a “carrying charge”.

Also, under the centralized plan, the dispersal of the treated water would be a “land dumping” operation, with the cost of the dispersal system not providing any benefit other than making that water go “away”. So the entire reclaimed water distribution system and the entire dispersal system would all be extra costs, instead of displacing irrigation systems that would otherwise be installed anyway. As well as wasting all that water, while the homeowners would purchase potable water to run their irrigation systems.

If, instead of implementing their development-scale conventional centralized system, the developers connected to the City of Dripping Springs system, it does not appear that their cost situation would be much, if any, better. The estimated cost of the “east” interceptor that would receive wastewater from Headwaters is $7.78 million (per the Dripping Springs PERP dated July 2013). While of course that interceptor would eventually serve other development, its major reason for being in the Dripping Springs plan is to incorporate Headwaters into the city’s proposed conventional centralized system. Dividing that cost by the planned 1,000 lots in Headwaters, the cost per lot is $7,780, by itself almost as much as the rough estimate for collection and treatment of the wastewater and redistribution of reclaimed water under the decentralized plan. This would be in addition to the internal sewer network, including several lift stations, within Headwaters. There’d also be a charge for buy in to the city’s treatment capacity.

Of course, this all presumes that the city could have that interceptor and the lift station(s) associated with it on line, as well as its treatment plant expanded, before the first house in Headwaters becomes occupied. As Headwaters has filed a preliminary plan for its first phase of 208 lots, that is an open question. Dripping Springs has not yet even released its revised PERP, thus has not even begun the permitting process at TCEQ, which can be expected to run about a year. If all goes well, that is; it could get longer.

Also note that it would be only those 208 lots, not 1,000 lots, that the developer could spread the buy-in costs over. But the entire $7.78 million must be put in the ground up front, along with the $8+ million for the treatment plant expansion. And the estimated cost for permitting is another $1 million in up front money. It will not be the developer who will be prevailed upon to cover all these costs, rather they will be covered by bonds, the payments for which will doubtless be spread over the entire city’s ratepayer base. So there is an aspect of social equity to be considered here too, as existing ratepayers will be required to help pay the costs incurred due to growth. Which, it has been asserted, will never pay back through tax revenues what it costs to install and maintain the infrastructure needed to actuate it, at least if that continues to be the conventional infrastructure.

A note about operations and maintenance. Understand that all of the decentralized concept systems would be under unified management, either by the MUD organized by the developer or as an integral part of the Dripping Springs wastewater system. While it won’t be belabored here, it is expected that the O&M costs of the decentralized concept systems would be less, perhaps significantly so, than for the conventional centralized system.

All things considered, the price of the decentralized concept system appears likely to be a sweet deal for the developer, even without taking into account the “time value of money” benefits of installing the wastewater system infrastructure on a “just in time” basis, to serve only the neighborhoods slated for imminent development. And because of the social equity issues, it would be a sweet deal for the existing citizens of Dripping Springs as well.

A WIN-WIN-WIN

Then add on the reuse benefits, displacing irrigation demand from the potable water system, which will further benefit all the citizens of the area by delaying, perhaps obviating, the need to implement a very costly long-distance “California-style” water transfer scheme that would greatly increase water rates. Altogether this is a win-win-win. So clearly it would be in the interests of Dripping Springs and the developers there to give meaningful consideration to a decentralized concept wastewater management strategy.

 

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9 Comments on “THIS IS HOW WE DO IT”


  1. […] water. « Water for DFW – Building-scale rainwater harvesting vs. Marvin Nichols THIS IS HOW WE DO IT […]

  2. Pliny Fisk Says:

    David

    Great stuff – comprehensive / understandable – one catchup statement and then a question

    just a few comments – First we at CMPBS are beginning to really cross the nexus of water and energy by incorporating cistern foundations that take the place of expansive soils while storing an immense amount of water for the same or close to the same cost of a regular foundation and – second these foundations become the heat sink for the heat pump – we are also going towards lowering or raising the water temperature in the foundation cistern through simple solar heating ( winter) and and nocturnal cooling ( summer) – I have presented this to the Resource Planning Commision, the CEO of AE, the AIA COTE, AGBuilding, Colony Park etc with not a whole lot of support or acceptance

    Question – on site wast water with an underground cistern in your case and ours is an interesting legal issue – because obviously the two cannot mix via underground flow of any kind seems that you might have the same problem with the part of the cistern in your diagram underground – how do you legally and practically solve the problem of never can the two mix – distance or supper sealing ar the only alternatives I see – look forward to your comment

    • waterbloguer Says:

      Hi Pliny. Thanks for the comments — good stuff. Re the question, a below-ground cistern would of course be sealed in any case — or else it isn’t a cistern, it’s a seepage pit. In any case, the ground around it is not “normally” saturated, so there would be no “driving force” for water outside the cistern to “invade” even if the sealing was compromised. So really a drip irrigation field receiving treated “waste” water — which is the only sort of “waste” water system a rainwater harvester “should” be using so as not to waste this water resource, rather to maximize its irrigation value — could be right next to the cistern and it would not create any real hazard. That’s the “practical” situation. As for the legal niceties, I’m given to understand that a cistern with its top above grade is not considered a below-ground cistern for purposes of applying the setback specified in the table in the state on-site rules. In any case, that setback is 100 feet, which is utterly ridiculous, and any “reasonable” regulatory body would be hard pressed not to grant a variance in consideration of there being no “reasonable” case that a hazard exists. Of course, many of our regulatory bodies fall short of being “reasonable” 😉 Thanks again for your thoughts.
      David


  3. A very comprehensive review as usual David. On the potable supply side of things, you say that the development would use water delivered by the Hwy. 290 pipeline from Lake Travis. Is that where everyone in this area gets their potable water? Are decentralized water supplies (individual wells or small communal well systems) not feasible due to water shortages?

    • waterbloguer Says:

      Thanks, Kelly. Local groundwater is under stress, and while it is used, developments of this scale are relying on imported water. I’ve been “suggesting” rainwater harvesting as a supply strategy, but (so far) it’s not getting much notice from developers, although many homes are being built “one off” using that as their water supply in this area.

  4. bshone59 Says:

    Essential information, explained well, great job!

  5. ty hall Says:

    I lived in the Caribbean for 2 years designing resorts and high end villas within the development. Rainwater harvesting was a key element in which we collected up to 30,000 gallons of water from the roof into cisterns built under the houses as part of the foundations. The water was used for drinking, washing clothes, flushing toilets and also for site irrigation. This is another element that should be considered as water gets scarce and more costly.

    Great article, well done and finally simplified that most of us can understand without too much technical expertise in wastewater management and engineering!

    All the best!
    Ty Hall
    landscape architect #1499

    • waterbloguer Says:

      Thanks, Ty. Glad it “came through”. Re harvesting rainwater, that’s been addressed in previous posts, in regard to whole house supply, and the next post will look at it as a component of stormwater management, to take some of the irrigation off the potable supply, in the context of a stormwater management scheme for this same neighborhood.


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