What to do with sludge?

2016/7/6 9:47:35

We live in a consumer’s society. We buy more, use more, produce more and eventually – throw away and dispose of more. The unwanted stuff, the undesired byproduct of our daily consumption, needs to be disposed of, taken away from our eyesight, our immediate environment and our comfort zones.

But we want to be more “green," so we recycle, compost, buy sustainable, eco-friendly, local, organic, and all natural products, but at the end of the day, we are all producers of sludge, aka "biosolids."

Population Growth and Waste

And there are more and more of us. The growth of human population is unsustainable. Like a bacterial colony on the petri dish, we continue to multiply, eating through the finite amount of “stuff” available to us until we deplete all the available resources and produce enough waste that the whole colony will collapse. The question then of what to do with the waste becomes the question of survival.

What do we do with the waste produced by the population growing at the increasingly rapid pace, consuming more and more resources, and producing increasing amount of hazardous waste?


What are Biosolids?

Biosolids is a toxic, hazardous substance. EPA considered it a hazardous material before it was rebranded as a “natural fertilizer," It’s the major byproduct of wastewater treatment: Solids removed during the primary sedimentation process and “stabilized” by drying, dewatering (decanter centrifuge then will be a desirable solution) and then digesting or heat treatment. Sewage sludge that underwent this treatment is called "biosolids" by the waste industry and is regulated under the Title 40 Code of Federal Regulations, Part 503. Biosolids are disposed of in landfills and incineration facilities, but the majority of them are land applied as “natural fertilizer”.

But let’s put the definitions aside and talk about why we consider it a waste and what the consequences are of this perception. Despite the enthusiastic assurances by the waste management industry of biosolids being a valuable and beneficial resource, this byproduct of our society is treated like a waste in need of the disposal. That should be accomplished in the least expensive way. Hence, at least 65% of US industrial and municipal sludge is land-applied as Class B biosolids.

The cliché "when life hands you lemons, make lemonade" strangely fits the point I want to make today. If we (and I mean “we” in a broad term; the society) continue to produce more sludge and then continue to dispose of it in the least expensive way, we will continue to create the sludge bubble, so to speak. It will burst one day, and it will not be pretty. It is time to change the perception from liability and a burden to resource and asset.

Quantifying the Value of Sludge

In a recent study, the scientists at Arizona State University evaluated different metals present in the sewage sludge and estimated the net worth of the sludge fraction. They reported the findings in the journal Environmental Science & Technology: there is as much as $13 million worth of metals in the sludge produced each year by a city of million people, that includes $2.6 million in gold and silver.

The city of Suwa in Nagano Prefecture in Japan is already recovering gold from sewage sludge — nearly 2 kilograms of gold recovered from every metric ton of ash left from burning sludge.

Another approach to turn the sludge into an asset is it’s potential as an energy producer. Wastewater treatment accounts for about 3% of the U.S. energy load and the process of stabilizing the sewage sludge to produce biosolids is where the demand for energy is the highest. If the biogas produced during the anaerobic digestion is captured and re-used, the wastewater treatment could become an energy producer rather than the energy consumer.

Anaerobic reactors are already in use throughout the world, producing sludge-derived methane that can be combusted on site for heat or electricity generation, it can be cleaned and sold to local gas providers or it could be used as a biofuel for vehicles.

Potential energy could also be gained from the thermal heat contained in wastewater for the use in heat pumps to heat residential buildings, as is already done in some countries with cold climates in Europe, such as Sweden.

Making Biodiesel from Sludge

Yet another economically feasible option to turn the sludge into an asset is the biodiesel production. A group of scientists from South Korea published the results of their study in 2012, where they demonstrated that production of biodiesel from sludge could be a profitable option due to the remarkably high yield of oil and low cost of the feedstock.

The scientists argued that the production of fuel from sewage sludge is superior to those from algae or soybean oils. The technology involves transforming lipids extracted from sewage sludge into the biodiesel via the thermochemical process under ambient pressure in a continuous flow system.

Our goal to achieve sustainability requires that we begin to look at our sanitation systems as a resource recovery rather than waste management systems. The subject of sustainability is a hot-button topic in today’s society, and as it applies to the issue of biosolids management, it should be evaluated not just from the economical and performance position. An evaluation of environmental, public health, and societal impact needs to be considered, conclusions should be drawn based on the most recent scientific and technological advancements rather than economical and legal ramifications.

By shifting today’s paradigm from how to quickly and profitably dispose of the waste to a new paradigm focusing on reduction of waste and what can be recovered from it, we could be able to escape from the increasingly toxic petri dish filling up quickly with the “beneficial” biosolids.