By: Yang Yan
One of the most important challenges in the modern world is to develop efficient methods to control environmental pollution. In the case of purifying wastewater, especially for the sanitary wastewater with low-concentration pollutant, there has recently been progress in making a solar-powered, recyclable and household device.
Water and wastewater facilities often make up the largest portion of a community's energy bill, accounting for up to 35% of typical US municipal energy budgets.  Because energy is required for all stages in the water treatment process, operating water and wastewater plants is very energy intensive and often expensive.
Heterogeneous photocatalysis technology was traditionally regarded as an efficient method for purification of wastewater in most cases.  However, when it comes to practical application, the single photocatalyst cannot efficiently purify wastewater without sunlight irradiation. In addition, as large amounts of wastewater may pass through the device so quickly that the catalysts may not have enough time to fully decompose the hazardous materials.
In light of this, a research group has come up with an innovative idea  – using commonly-seen activated carbon (AC) as a reservoir for the hazardous martials. It works as follows:
1. The device is installed in the drain tube
2. As wastewater flows through the device, the activated carbon absorbs hazardous materials
3. Every 20-30 days, the user takes out the device and expose it to sunlight
4. Sunlight and photocatalyst together decomposes the hazardous material into CO2 and water
5. The user put the device back to the drain as if it were new
This simple, energy-free way of treating wastewater is a lot of advantages and can be useful in various ways:
1. It requires no energy, so it can be used in rural and developing areas where electricity is not readily available
2. The treatment process is spread into millions of households, so there is no need for large treatment plants which can be an economical burden and health threat to the city.
3. All the materials used in the device is cheap, easy to make and non-toxic.
Of course, at present, this is just a prototype and unavailable for mass production. The research group’s sample device only retains 23% adsorption capacity after 4 runs 48 % that of photocatalyst (Figure 1). Still, this idea is promising and worth further researching.
 Hoffmann M R, Martin S T, Choi W, et al. Environmental applications of semiconductor photocatalysis[J]. Chemical reviews, 1995, 95(1): 69-96.
 Yan Yang, Lu Wenjie. Combined Technology of Adsorption and Photocatalysis for the Purification of Wastewater. 2017.
Figure 1. Long term adsorption and photocatalysis experiments  (Lower bars means better performance)