20-0114 v3.X Cooling Tower Effluent
For our development, we are working through ways to meet the Net Positive Water imperative, attempting to meet demands within the carrying capacity of the site and to mimic natural hydrological conditions using appropriate water management systems. This dialogue post is our request to be able to discharge the effluent from our cooling towers to the municipal sewer.
• The project is a shopping centre with a predicted high intensity usage dependent on visitor numbers (rather than being more predictable / static as might be expected for say, a residential or office project). Note - visitation to our building is estimated at around 3 million visits per year, i.e. the more popular the shopping centre, the higher the water demand and the more water to manage on site!
• The project is in an existing urban area and therefore we don’t have a nearby field into which to simply release water. We also do not have the ability to inject water into the ground. We therefore require other ways to remove water from the site and try to maintain a balanced hydrological cycle.
• We have installed a mechanical wastewater treatment system to treat captured rain, stormwater, and after in-building usage. This recycled water will be used for non-potable uses, including toilets, irrigation, and a carwash, as well as cooling towers. We will likely have excess recycled water due to the above (high visitation and the non-rural environment).
Integration with energy strategy
As part of the low energy strategy and in an attempt to utilise as much recycled water as possible, the project has opted to use a water-cooled system with cooling towers, which is an effective way to evaporate recycled water whilst radically improving building-wide energy performance through the HVAC system.
Cooling tower blow-down
Up to 95% of the water used in cooling towers will be evaporated in the process of HVAC operation. The remainder is high mineral concentration system water that must be discharged as “blow down” to prevent scaling of the system, i.e. after 95% of the water has evaporated, the remainder is a sort of concentration of “brine”. Unless a portion is bled off and replaced with fresh water for dilution, the salinity of this water will progressively increase as more water is recycled and kept within the system, Highly saline water is unsuitable for irrigation and will usually degrade plumbing infrastructure.
We have explored local uses for the brine: surface water discharge, deep well injection, and meeting with agencies and authorities, who have informed us that they are in the process of developing a sustainable reuse program for all wastewater that comes through the municipal sewage network (e.g. applying the sludge to agricultural land). We do however want to confirm our approach irrespective of the future sustainable reuse by the municipality.
Essentially, we see three options for disposal:
1. Re-use the brine in the shopping centre.
2. Pump the brine into a truck and drive it off-site.
3. Send the brine to via the existing sewerage network to the municipal treatment plant.
In the case of our project, Option 2 is the only practical and environmentally-sound option. This is because:
• The site doesn’t have any use for blow-down, hence Option 1 is not possible.
• Trucking the brine offsite as per Option 2 is not sensible, as it would result in trucks attending the shopping centre to connect to a pump-out point and multiple driving trips to a licensed disposal point, which is likely to be the same sewage treatment plant that the brine would go to if discharged to the sewerage network. The result is excessive greenhouse gas production, traffic and car park congestion, and infrastructure costs without any known benefit.
• Option 3 allows the brine to be managed at the municipal sewage plant, where it can be treated, composted, and / or transported to a different industrial facility for beneficial re-use. These processes only work at a municipal scale. The brine itself will be only a fraction of what will be treated by the on-site mechanical water treatment system and able to pass through the sewer network. The local authority are prepared to accept it under their normal trade waste policies.
- Project water will be captured, stored, treated, and used for non-potable uses in the building, e.g. cooling towers, irrigation, toilet flushing, and car wash.
- A piped connection will be provided for excess wastewater that cannot be accommodated on site, delivering it to the municipal network where it can be managed more effectively at a larger scale.
- To account for this, we will include in the project energy production the pumping energy required to move the blowdown from the site to the municipal system.
We would appreciate if the ILFI could confirm the logic and validity of our approach.
AnswerThe Dialogue Post requests that the project be allowed to send concentrated brine effluent from its cooling towers to the municipal sewer through an onsite connection.
Because 1) the project is required by the municipality to have an onsite sewer connection, 2) there is no option for beneficial use of the brine onsite, 3) treatment within the municipal sewer system offers the potential for beneficial use in the future, and 4) delivery via the onsite connection is environmentally preferable to trucking between the project site and the municipal treatment facility, the proposed approach is acceptable subject to the conditions listed below.
However, the post also indicates a general expectation of excess wastewater and an intention to send that to sewer. The team has provided clarification on this point, including detailed calculations and specification of a hierarchy of sources for onsite non-potable demand that prioritizes use of recycled wastewater and rainwater. Because the project is treating wastewater, including sewage, and maximizing its use for non-potable applications onsite, and because the site geology is not suitable for infiltration, and the Authority Having Jurisdiction does not permit treated wastewater to be released beyond site boundaries, it is acceptable in this specific circumstance to allow wastewater that cannot be managed onsite to be discharged to sewer subject to the conditions identified below.
Supplemental information supplied by the team also indicates that the hierarchy for non-potable uses includes potable water as a last resort. When being supplied by recycled site water, both municipality requirements and reliable operation require that systems essential to building function, including toilet flushing and the HVAC cooling tower, have back-up sources. These systems and other non-potable demands will be satisfied first by water recycled from onsite uses with collected rainwater being the primary back-up source. Potable water is the second back-up source and will only be used if the first two sources have been exhausted. Given project and site constraints, the approach is consistent with the Imperative intent to treat and use all water onsite. It is acceptable in this specific circumstance to allow potable water as a secondary back up source as proposed, subject to the conditions identified below.
The project's wastewater system as proposed is allowable, subject to the following conditions:
1) The hierarchy for non-potable supply is structured to maximize use of recycled site water (wastewater treated onsite) and use potable only as a last resort when the onsite recycled site water and primary back-up (rainwater) have been exhausted. This hierarchy must be incorporated into the narrative and diagram system descriptions in the required documentation and into the system’s operation and maintenance materials.
2) Meters are installed to measure flows in the following locations:
- all connections to sewer
- backup connection between potable supply and non-potable use
- rainwater storage and distribution
- treated wastewater distribution
- discharge to retarding basin
3) Flows are monitored and adjusted as necessary to maximize re-use of recycled site and rainwater and minimize use of potable water for non-potable applications.
4) Advocate to authorities having jurisdiction to facilitate installation of systems that both treat water as a precious resource and minimize chemical and energy inputs, including specific requests to:
- Develop a municipal-scale system for beneficial use of treated wastewater
- Develop a sustainable reuse system for byproducts of municipal sewage treatment (e.g. applying the sludge to agricultural land).