The Wastewater Unicorn - No More Biological Solids.

Ever since I joined the world of wastewater treatment in 1992, the reduction or elimination of biological solids produced by secondary treatment has always been a holy grail, or being more on trend - the unicorn. As a nuisance by-product requiring treatment and disposal, biological solids added to the system cost and was always the most unloved of process operations. It’s hard to thicken, it’s hard to treat further, hard to dewater and can give rise to odour issues because of its active nature (especially when mixed with primary solids).

I’ve seen various proprietary processes, from very long sludge age activated sludge systems to physical disintegration processes to achieve a reduction in the biological solids. Unfortunately, these come at a cost. Nor have they been entirely successful in their application with only a handful of plants in operation globaly.

In recent times, the use of a hydrolysis processes upstream of anaerobic digestion has released value from secondary solids albeit at a significant investment cost. This has certainly helped lessen the impact of secondary biosolids by increasing biogas production and reducing the mass of biological solids left for disposal. Further advances in energy recovery such as gasification or pyrolysis are also beginning to make inroads into the industry.

However, returning to the initial premise that secondary biosolids are still an industry burden and reducing production is of financial benefit, let’s examine where these financial benefits would reside should we be able to avoid production altogether.

·       Lower pumping and storage costs. Generated biological sludge must be pumped and stored, usually with some form of mixing and potentially odour control. There is therefore an energy cost to be avoided.

·       Improved thickening effectiveness. Biological solids are difficult to thicken and have higher polymer demands over primary solids. Cost savings here can be significant by avoiding this altogether.

·       Increased digestion efficiency. There is less time needed for the anaerobic digestion of primary-only solids, reducing digester capacity and subsequent mixing and heating requirements.

·       Better dewatering. The subsequent digested content has a both a lower mass and a lower volatile solids (VS) content. As a result, there is both lower polymer consumption per kg and a lower total quantity to be dewatered.

·       Lower transport and disposal costs. The lower VS content will lead to improved dry solids content for the same dewatering effort, reducing water to be transported off-site.

·       Reduced ammonia demand. The digester return liquors from only primary solids will have a lower nitrogen content and therefore a reduced concentration of ammonia being returned to the process, reducing aeration demand.

Even without considering the additional savings made on reduced pumping costs between process units (e.g. thickening to storage, return activated sludge etc,) and equipment maintenance costs, we would expect to reduce the operational costs by approximately 30% for a typical treatment plant by removing the biological solids.

This reduction allows for the loss of power and heat production that would have been realised through the digestion of the now absent secondary solids.  Spread that across the globe and that’s a significant reduction on the operating cost base, not to mention the potential greenhouse gas reductions associated with treatment, transport and disposal. Of course, all of this is subject to the idiosyncrasies of each application, but nevertheless the opportunity for significant operating and capital cost savings is available to us.

By reducing the secondary solids load onto the digestion process, we can release at least 40% of the available digester capacity (for an existing site). This newly available capacity can be re-directed towards importing other digestible waste or providing additional capacity for wastewater sludges. This can deliver capital savings to accommodate catchment growth or make a difference in the sludge deregulation debate (specifically for the U.K).

But let’s just take a minute here.  Previously I’d expressed that we haven’t seen major success in delivering biological solids reduction so is this unicorn entirely and utterly mythical, and this is indeed just a process engineer's pipe dream? 

Well perhaps not. Previous attempts have always been based upon finding mechanisms to destroy the produced biological solids, rather than preventing their production in the first instance. Many would consider this be against nature itself and therefore impossible to achieve so have not pursued this recourse. I would not argue against this in the context of today’s methods for hosting microorganisms we use for biological degradation in modern day treatment plants. Insanity, after all, is defined as doing the same thing repeatedly and expecting a different outcome. I am, I believe not insane.

It was Microvi founder, Fatemeh Shirazi, who recognised that it was the organism’s microenvironment that provided the key for a new paradigm in performance and solids production, and focused efforts on understanding and recreating new habitats which could be introduced into modern reactors. Following years of research and development and numerous prototypes (just like Dyson and his cyclone vacuum cleaner), a successful outcome was finally achieved, that is the understanding of how to create these environments using materials science and the benefits that this achieves. Microvi call this breakthrough MicroNiche? Engineering (MNE) and consider it to be a platform for many possible applications. By creating these perfect environments and selecting the right organisms, Microvi can produce a biocatalytic polymer composite that can be used to treat water and wastewater very effectively. The organisms inside these composites behave very differently to those normally associated with biological treatment, and it’s these behaviours which give rise to a series of beneficial outcomes.  Chief amongst these benefits is the ability to hold a stable population, with no net growth of biomass, with energy being redirected by the organisms towards cell maintenance.  There are other benefits such as increased ability to withstand toxicity and have a higher concentration of organisms per volume of reactor versus other systems, meaning shorter residence times and smaller systems.

Is this just hokum or snake oil? After all we’re going against our perception of nature and a century of previous industry thinking? Well no, the specific mechanisms involved aren’t novel or unique in nature, (but they are in Microvi’s application). Behaviours such are quorum sensing and autophagy are employed by the high density bacterial community, with the organisms focusing on remaining fit rather than reproducing and dying fast. The terminology used to describe this state is non-growing, metabolically active.  We have observed this mechanism in action in full-scale installations and demonstration pilots giving real world confidence to Microvi’s benefit claims.  

By identifying and exploiting this white space (the microenvironment), Microvi have developed a technology that could radically alter the landscape of current and future water and wastewater treatment—the unicorn may have been found after all.