[Natalia Bourenane]

@BiogasWorld_Burak

[Burak]

There are many companies that are advertising a few technologically similar solutions. I am attaching a table I prepared about 5 years ago which lists these categories in basic.

In very broad summary there are:

1- Evaporators: These technologies are viable if you have excess heat available. They are very costly to run and their final products carry out all the impurities in the source water. Evaporators are very good technology if you have CHP where you don’t utilize the heat.
2- Struvite precipitation: Struvite is a very viable option for recovering nitrogen and phosphates from the waste water. It uses magnesium to form MAP (Magnesium Ammonium Phosphate). Main issue of struvite precipitation is the high chemical consumption and operational difficulties. However it is very good quality recovered product.

3- Anammox: Anammox, anaerobic ammonium oxidation is a method that is mostly used in WWTP plants to remove ( not recover) Ammonia from the waste stream. It has limited application in biogas field as far as I know it. It is energy efficient.

4- Ammonia strippers: Ammonia strippers is one of the most common technology available. To quote: Ammonia stripping seems to be one of the preferable technologies. Ammonia stripping is using relatively cheap chemicals however, it often has clogging problems in the stripping columns. Main advantage of ammonia stripping is that it recovers a standardized nitrogen fertilizer product which can be also used to enrich other digestate fractions, making them more profitable. (Teodorita Al Seadi, 2013) Ammonia strippers are hard to use, expensive and something operators hate to use.

5- Membrane filtration: Membrane filtration and several other methods can be used together (such as polymer addition etc.) to reach nitrogen discharge levels that are acceptable. Unfortunately that requires about 3 RO in series which would require extremely high maintenance and total solids concentration that would be nightmare to deal with.

6- Hydrophobic membrane stripping: This is what we do at NPHarvest. Our system doesn’t do filtration. We don’t use heat or high pressure. What we do is a two step process to recover. We first increase pH to 10 by using Ca(OH)2. This has two effects. First ammonia becomes gas and also Calcium acts as a flocculation agent. After that we do a ballasted sedimentation to recover P. Process then continues to our patented design hydrophobic membrane modules. In these modules water just gently flows around and it is not pushed. Our hollow fiber membranes circulate acid inside and due to concentration difference ammonia passes inside the membrane and form ammonia salts with the acid, moving away with the circulation. Therefore there is always a concentration difference and passive diffusion happening. With this system we can reach up to 90% recovery. Our nitrogen salts are high purity and our phosphorus product is a combination of Ca, C and P. Note: Ourd effluent has still pH10 and depending on the application a post treatment would be required for disposal.

There are also few alternative treatments such as Ion Exchange etc. however I have not seen much application of these in the field.

There are two main issues in nutrient recovery:

1- Opex is too high. NPHarvest aims to be profitable if there is a disposal costs factor. Most of the treatments are net negative to the plant owners.

2- Recovered nutrients could be not usable as fertilizer. Plastic and heavy metals are main contaminants. Many countries don’t allow polymer flocculation that most processes depend on. Therefore end products are not viable as fertilizers.

Nutrient recovery for biogas plants is an urgent problem that has been known over a decade but kept ignored. Now as the EU tightens Nitrogen discharge levels (170 kg/h/year as it is), liquid digestate is coming a big cost issue for the biogas plants. I am sure rest of the world will be following as soil pollution and eutrophication is a big environmental concern globally.

I am happy to answer any other questions.

[Ryan]

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.

[Gururaj]

How high pH digestate will help soil, in high pH digestate we are losing nitrogen in the form of ammonia? At low pH liquid fertilizer is good nutrients mobility. What is best pH value when we are applying digested on soil as liquid fertilizer.

[Ryan]

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.

[Gururaj]

Thank you Ryan,

Any specific study papers on this subject, like nutrients mobility of soil, form of nutrients in digestate, effect of PH, land area VS application volume of digestate, applied benifits on soil. Please suggest any research papers or experience write-up if any.

[Ryan]

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.