Wastewater Heat Recovery an Increasingly Hot Topic

Wastewater is not something that frequently crosses the average person’s mind. Yet ask anyone their opinion on the topic and they will give you some basic characteristics: Wastewater mostly contains a variety of wastes from residential, commercial, or industrial processes. Few people acknowledge that large amounts of wastewater contain recoverable, valuable components such as nutrients, irrigation water, and heat.

The inherent wastefulness of single-use water systems is being addressed in a number of ways. Systems like purple pipes utilize gray water (i.e., water that has been used but is not contaminated with sewage) for things like flushing the toilet and watering lawns. Gray water comes primarily from sinks, showers, and bathtubs. Purple pipes are becoming increasingly popular, especially in drought-ridden areas like California. In early 2015, San Diego began working on plans to expand its purple pipe infrastructure in an effort to reduce the need for imported water. Purple pipes can be very beneficial for reducing total water consumption. However, these systems currently do nothing about capturing heat waste.

Every day, billions of gallons of hot water go down the drain as hot showers are taken, dishwashers are run, and food is cooked. Most of this heat is lost entirely, while fresh water is heated up in isolation. In fact, between 80% and 90% of the energy used to heat household water goes down the drain. Domestic water heating also accounts for 15%-25% of household energy consumption, so wasting this heat represents a solid chunk of inefficiencies. International Wastewater Systems (IWS), Power-Pipe, and other companies are working on solving this problem by installing heat exchangers in the wastewater system.

Entropic Inevitability?

Similar to an energy recovery ventilator (ERV) in an HVAC system, heat exchanging systems for wastewater are typically most economical at large volumes, so they are currently installed mostly in commercial and industrial buildings. These systems transfer heat from wastewater directly into the fresh water stream without any direct contact between the waters. IWS has a patented system suited for large buildings (the SHARC) that can reduce energy costs by 30%-85%.

Other companies such as Power-Pipe sell individual components for residential systems. One Canadian company, Watercycles, markets a drain water heat recovery system for household use that utilizes water from the shower drain to preheat water from 40°F to 70°F before it hits the water heater. This preheating has the potential to double the output of the heater itself, reducing energy costs by half. Heat exchangers are not as common in households yet, primarily due to high investment cost. However, they are increasingly popular in high occupancy commercial properties like apartment complexes and hotels.

Wastewater is gradually becoming viewed as a valuable resource. Reducing the amount of water, nutrients, and heat that go down the drain is not only necessary, but valuable. As more buildings adopt purple pipes and heat recovery systems, this value will be realized.