01 February 2012

Delivering water quality

It's tradition to take drinking water from a river above a city and discharge the wastewater downstream, for the simple reason that one wants to start with clean water and dump dirty water off one's property.*

This tradition does not help very much when there are other cities downstream (as their inevitably are), cities that get wastewater into their drinking water intakes.**

Many regulations have been implemented to address this problem. Most of them require treating the wastewater, to reduce the burden on downstream neighbors. But those regulations are not always perfect in terms of delivering results. It's possible for a city to be "in compliance" and yet send nasty stuff -- stuff it would not want to see in its drinking water -- down to neighbors.

There are two ways of dealing with this problem of asymmetric information (the city knows what it discharges) and incentives (it can comply with the law knowing it's not the right thing to do).

The first is to require that a city take IN water downstream and discharge OUT wastewater above their intake. That makes it easier for water managers to find ways to discharge cleaner wastewater, but it's also expensive when pipes are already in place.***

The second is a form of "regulatory piping," i.e., a requirement that downstream water a city be higher than water quality upstream of the city. That regulation can be enforced by measurement at both sites.

Bottom Line: If I don't know what you are doing, at least I can make sure that you pay the consequences for doing the wrong thing.
* You can see the same thing when people sweep their property, dumping dust, etc. onto the street.

** This fact shows how the "toilet to tap" debate over recycling wastewater for local use is so silly -- it's just a "shorter loop" version of what happens already. I heard one time that San Diego's drinking water has been through an average of seven toilets.

*** At least one city (Aurora, in Colorado?) already does this, but for reasons of quantity; it can take more water out of the river downstream if it adds its wastewater upstream.

4 comments:

Jay said...

Taking water out upstream and discharging waste water downstream also takes advantage of gravity for part of the energy used in system.

Waste water, especially, takes advantage of gravity flow where ever possible. Drinking water is generally pumped to the top of a hill or water tower. The lower the intake, the higher the pumping costs.

But I agree, discharging upstream certainly internalizes the externality and provides very clear incentives. It's a question of thinking like an engineer or thinking like an economist.

David Zetland said...

@Jay -- that's why I suggested the virtual solution of meters to represent where the intake/outflow should be. I agree that it's best to take advantage of gravity.

Richard Scott said...

Aurora isn't the only place doing this for quantity reasons (cant remember other examples of the top of my head, but there are plenty).

But your quality solution is impractical. As Jay pointed out, rivers (as a rule) are following a gravity path past a town. The insitutional costs in reengineering a sewerage network to run uphill would be immense on a capital and operational costs basis

And your virtual approach is flawed - the effluent is but one contribution to the change in water quality as a river flows through a town. Stormwater runoff, pollutant infiltration and the nature of the stream bed are also going to be big influences. You can't expect the water utility to have to clean up all those unrelated externalities.

So the simple approach of regulating sewage treatment plant discharge looks to me to continue to be an efficient and effective way to treat the externality caused by the effluent. (And dwell time in a river is an extremely effective way of treating many human-sourced contaminants in water)

David Zetland said...

@Richard -- agree about point source reg, but this idea is more relevant to water shed/catchment manager/agency