Ryan
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The capital and operation costs are a huge hurtle to overcome for sure, as Burak noted. As well as the regulator requirements on what can be done with the nutrients once recovered.
There are Algal Biofilm systems, where algae grow on a membrane utilizing the nutrients from the liquid phase. The algae is then harvested and land applied. These systems do have mechanical components but generally do not use any additional inputs like polymers.
Where is possible, direct land application of liquid digestate is a simple, and likely more cost-effective solution to recover the nutrients.
May a pose an alternative to that question… What is the most sustainable solution to digestate management?
From my perspective, that is land application as a fertilizer / soil amendment. This may not be feasible for all situations where further processing is needed to turn the material into something that could be bags and sold like compost. But in many situation, especially for on farm systems, that it the most desirable use. In someways an AD system is just borrowing the nutrients to control the production and capture of CH4, and those nutrients are still highly valuable for use as a fertilizer.
As noted, beneficial reuse of digestate as a fertilizer / soil amendment provides a source of N that is more available to plants, reduced pathogen content, reduced ordour, among others. Some additional benefits are tied to the source of the organic material going through the digester and the typical method those materials were managed. For example, a SSO facility or a comingled system can divert organic from disposal in landfills, converting them into a form more suitable for land application as a fertilizer. When used by an industrial operation, like a food processor, processed organic waste from a digester could provide additional opportunities for disposal and cost reduction for the management of the organic wastes generated.
There are some things that need to be considered, include the total nutrient content of the digestate, especially when utilizing ag source materials such as manure with non ag source organics on farms. The controlling nutrient when applying manure to ag lands it typically P, for comingled systems that often changes to N being the controlling nutrient. That change can impact the acres needed to sustainably manage the digestate.
The type of farm and existing waste management system also need to be considered (the physical/mechanical part). As noted, poultry liter presents a unique set of challenges that need to be managed for optimal gas production (the chemistry part), but without consideration to the existing waste management system on the farm and what changes maybe needed, it ultimately won’t matter that the chemistry can be optimized, if the cost to change the physical/mechanical is too great.
Throughout NA, rules governing the application of digestate, or any organic material/waste, to ag lands are set by the state/province, depending on the classification. These rules are set with an understanding of nutrient mobility, plant utilization, pH loading, etc., usually accompanied with a soil sampling program to monitor past applications and to plan future applications. Generally, there is not additional value placed on digestate over other ag source materials such as manure, with the benefits being the nutrient content, organic matter, and in some regions the water within the liquid phase is used to offset irrigation water needs.
A pH range between 6 to 7 is preferred, but most soils and crops can manage a range of 5.5 to 7.5.
Depending on the soil type, crop plan, etc., liquid digestate with a pH outside of that range can be managed in the field with BMPs, which will require additional cost.
Ag source materials generally have less gas production potential and therefore less financial return potential, but on digesters built on farm do offer advantages for the management of digestate, as farms will likely have storage availability, land available for digestate application, and hauling equipment to get the digestate to the fields for beneficial reuse as fertilizer and soil amendment.
I’m not sure I would say ADs with ag source materials are less interesting, just that there are additional challenges to achieve a return on investment. Farms in Ontario, on average, tend to be smaller than farms found in other regions of North America, and obviously generate less ag source materials suitable for use in an AD system, making a stand-alone system less economically viable. Supplementing with off farm organic will help increase feedstock volume, as well as provide greater gas production potential than what is typical of ag source materials, but gaining access to a reliable, steady supply can be challenging. Clustering farms together for a single AD is a potential option, but again, the size and number of farms, in addition to the distance between farms presents a different set of challenges, among others.
Ag/farm-based systems do offer some benefits with the regulations around managing digestate as an ag source material vs a non ag source material for beneficial reuse as fertilizer and soil amendment. This can increase to viability of a mixed system, but there are some things that need to be considered such as available land based, storage, transportation, etc.