A tale of true city needs
About 200 miles inland sits the Reno-Sparks metropolitan area, population about 500,000. As one of the nation’s first cities to confront the “no more water to grow” issue, the city has spent more time than most looking decades into the future to identify reliable new water sources. In fact, water managers looked 100 years ahead and hundreds of miles in every direction as a part of a long-term strategic plan. And the reality of a truly finite supply hit home.
In 2008, the city engaged engineering firm Stantec Inc. to further the notion of indirect potable reuse (IPR). Conceivably, IPR could greatly extend the useful life of the existing supply, provide long-term reserves to tackle future droughts, and potentially allow growth to continue in northern Nevada for decades beyond expectations.
There was just one problem: the pharmaceuticals, endocrine disruptors, and flame retardants in wastewater.
This molecular cocktail of chemicals complicated the ability to achieve sufficiently high treatment levels. No conventional or advanced treatment processes had ever addressed contaminants of emerging concern (CECs). Regulators would never permit water laced with these substances, even in infinitesimally small amounts, for human consumption if the source was originally wastewater.
In response, we envisioned a unique treatment train using a combination of new and existing technologies that would meet drinking water standards and eliminate CECs without concentrating them into a dirty stream in a location with no affordable disposal options.
The city agreed to facilitate a two-year pilot demonstration system at the 1.5 mgd Reno-Stead Wastewater Treatment Plant to test the methodology’s effectiveness on 490 contaminants representing an entire suite of organic and inorganic elements. Completed in 2010, the results have been validated, documented, and peer-reviewed: 10.7 gpm of treated effluent meets or exceeds both goals. The city funded the $1.2 million project independently with minor support from a neighboring municipality and the nonprofit community.
Technically speaking …
The technology behind Reno’s approach is nonproprietary. Here is a semi-simplified breakdown:
1) Following filtration, ozone is used as a strong oxidant to break complex-structured contaminants into simpler-structured contaminants. Because some remain carcinogenic, the ozonated effluent is passed through a biological activated carbon (BAC) filter.
2) The filter has a very high surface area for microorganism growth while also adsorbing some contaminants. By allowing for microorganism growth on the filter medium, organisms that can survive on the available substrate (oxidized end-products) are naturally selected. The use of a biologically active carbon filter is similar to groundwater remediation projects whereby specific organisms are selected for clean-up activities.
3) Ozonation produces bromate and N-nitrosodimethylamine (NDMA), both of which are regulated. Bromate is mitigated with ammonia and peroxide; NDMA is removed by the biologically active carbon filter.
This process uses about 25% of the energy that would be consumed by RO without wasting any water. All effluent is refreshed for reuse.
It doesn’t, however, remove salt.
Over time, salt will slowly accumulate in the water supply. If salt becomes problematic, a small sidestream RO unit can be added to peel off and treat just enough flow to maintain salts at the desired level. This would consume an estimated 33% less energy than fullstream RO and leach fewer salts from soil, potentially eliminating the need to add lime to protect process piping from corrosion.
Generally, inland application of the ozone-BAC approach for CEC removal requires roughly one-third RO’s capital investment because zero-liquid discharge technologies must be incorporated to treat the RO’s dirty stream. If, however, the stream can be discharged to an ocean, capital costs are about two-thirds RO’s.