Just as “alternative conveyance” is another word for “peripheral canal,” so “sewage sludge” has become “biosolids.” At last night’s CAG meeting, we talked about what we’re going to do with all of it, whatever it’s called. Currently, the WPCP devotes almost 800 acres of the plant lands to the sludge lagoons and drying beds that take out most of the water from the solid waste left behind by the three-stage water treatment process we use. This stuff, call it biosolids or dried sludge, spends four years out in the fresh air and sunshine, after which it is used as “alternate daily cover” for the landfill across the street. The current process is essentially free, in that it relies on solar power to evaporate the water (there is some pumping involved) but it uses huge tracts of land, is responsible for the stink, and depends on a landfill that is due to close within the time frame of the plant’s upgrade. It’s time to come up with a new destination for the biosolids.
You know already how proud I am of our sewage treatment plant, and I am also proud to say that it produces exceptional biosolids. That is, Class A biosolids of Exceptional Quality, which means that they conform to strict rules about pathogen levels (the Class A) and metal concentration (the Exceptional Quality.) Throughout California and the US, biosolids are used as soil amendments, and considered as one element in the cycle of growth and consumption of nutrients, the prospect of making more such use of our own biosolids is very appealing, especially given my agricultural bent. But if we use the other term, sewage sludge, we get to the fact that it cannot be used for organic agriculture (although to be fair, sludge is not the same thing as biosolids. Sludge comes straight out if the pipe, where biosolids have been “dewatered” and, in San José’s case, had the pathogen level cut near zero.) Further, many agricultural counties in California have banned the import of biosolids from other regions. Dale Ihrke, the plant manager, said last night that it seems more a case of rural counties’ not wanting to have to deal with the waste of the large urban centers, rather than a prejudice against biosolids per se, but the effect is the same. The plant produces many tons of biosolids, and to use them with any kind of efficiency, we’d have to load them on trucks and ship them elsewhere.
So, attractive a notion as it is, we’re probably not going to be using much of our own biosolids, Class A and of Exceptional Quality as they are, to build the soil of urban farms in Santa Clara County, or anywhere else in California.
The Plant Master Plan upgrade process is predicated on the idea that a big chunk of the plant lands will be developed to offset some of the $1 billion cost of the upgrades, and reducing the footprint of the drying operation is a key factor in that. Smaller technologies include centrifuges or belt presses for the “dewatering” and greenhouses or heated chambers for the drying. All of these will take energy, energy we’re not currently having to pay for with our passive solar processes. But the same energy content that makes biosolids usable soil amendments is also available to us in other ways, and it turns out that sewage sludge has roughly the same energy content as low-grade coal.
Further, systems that mechanically remove water from sludge and then incinerate the biosolids just about break even in energy use and production. But not monetarily; they are very, very expensive, with much of the cost coming from meeting air quality regulations. But they reduce the volume of biosolids by 90%, and the resulting ash, as well as being absolutely sterile, is also usable as a component of concrete.
So there you have it. I am now a firm believer in incinerating sewage sludge. My ideal Santa Clara Valley would use this rich source of nutrients to build the fertility in the soil of the many farms that would dot the landscape. But the valley I live in could do worse than to keep dealing with the biosolids on site, recover some energy from it, and sell the remains.