By Sally Brown, University of Washington
This is, after all, the September Issue of the Biosolids Library. For those of you who are regular readers of the library but not of Vogue Magazine, you may not get the reference to the documentary film about the making of the largest and trendiest issue of the year (click the picture above for a summary). If you want to talk about trends in our line of work, then food waste – the topic of this month’s library – is all the rage. It is our equivalent of the September issue. It is the trend this year and likely for a few years to come. Food waste is making the headlines in the New York Times, the Huffington Post, in municipal governments and even in the general public. I have even been asked to blog about this for the Huffington Post (http://www.huffingtonpost.com/author/sally-brown2).
Food waste also represents a gigantic opportunity for wastewater. While others may see rot, slime and smell, those in the know see high methane generation potential, increased volatile solids reduction, tip fees, and power to the plant and the grid if material is handled properly and proactively. If not, that same rot, slime and smell can translate into higher BOD going into aerated secondary treatment, FOG that clogs pipes rather than generates power, and plants approaching capacity limits. The same basic principles apply to compost facilities – if handled well food waste is an opportunity for products richer in nutrients and so more valuable on the way out with higher associated tip fees on the way in. If not handled so well… likely no need for details on that one.
The library starts with an article on just how much food waste is generated annually in the U.S. The per capita number is staggering – over 100 kg per year per person on the consumer level. That is not counting loss prior to us losing track of what is in the fridge. The study breaks this up into different groups of food including meat, poultry and fish, dairy and vegetables. The dollar value loss is very high and the potential for increased efficiency seems clear. Even with more efficiency- that still means a lot of waste.
The next three studies compare the costs and benefits associated with how to deal with those wastes. The second article is co-authored by Mort Barlaz from NC State. He has traditionally been a landfill guy and not much of a compost guy. In the article different options for commercial food waste management including landfilling, anaerobic digestion, composting and combustion are compared using life cycle assessment. Landfilling comes out the worst here with AD and WTE (waste to energy) as the best. Composting comes out in the middle. It is important to note that in this article they consider high percentages of contaminants in the food waste – up to 40%. The authors also assumed fugitive methane emissions of 3% from controlled digestion. Assumed moisture content is not reported. While they consider fertilizer offsets and allude to soil carbon sequestration, they also consider metal recovery from WTE as a benefit. His high ranking of WTE also likely stems from high BTUs for the contaminant fraction of the food scraps. So very high rates of contamination and likely some false assumptions color this, but realize that even with this, AD comes out on top. From here we go to a study commissioned by OR DEQ led by Jeff Morris. Here we (I helped out on this one) took a very Oregon centric view. We also included a major section on soil benefits of different options. Biosolids reported benefits were used instead of digestate associated benefits as the two are comparable and there is a lot more literature on biosolids than on food waste digestate. I recently had the opportunity to tour a food waste digester and I held the dried fertilizer pellets in my hands (http://www.earthpower.com.au/creating_green_energy.php). They bore a striking resemblance in analysis, smell and feel to the pelletized biosolids that I have seen.
The third in this series is by David Parry, a local here in the Northwest. The Parry article is from Biocycle, in other words it is short and to the point. It covers much the same ground as the last two but also considers in sink digestion and costs of the different options. In sink digesters are bad both for the environment and for the bottom line. Bringing that BOD into the front end of the plant uses rather than generates a lot of energy. We also considered that option in the OR study and reached a similar conclusion. Composting here is the lowest cost alternative and also has a relatively low carbon footprint. It has a large physical footprint, which makes it difficult for some municipalities without access to land. Here again, AD is the winner from a GHG perspective. However capital costs are high.
That brings us to the last article, and it also brings us home. Here the topic is anaerobic digestion at wastewater treatment plants. Co-authored by Maile Lono-Batura and Ned Beecher and from Biocycle magazine, it may be something you have seen before. If not, you should give it a look. Many plants, particularly the larger ones have existing and underutilized AD capacity. Taking food waste into these facilities, at the back end of the plant, offers multiple benefits. Reduced stress on the front end, extra energy, more resilient digesters, potentially better volatile solids destruction, and tip fees. Many facilities are already doing this or have started talking about it. The food waste trend is here and so is the time to capitalize on it, both for your bottom line and for the environment. If you do, I might even mention you in my blog.