Alfredo, attached is a paper that discusses the conversion of organic waste (like digestate) into industrial chemicals and protein.  My company is currently discussing working with the anaerobic digestion industry in Europe as a means to change the economics of the AD process and deal with the issue of the disposal of digestate,  The MEF process permits the removal of the water and changing the economics of the process.

Have you considered growing floating aquatic plants like duckweed on the liquid portion of the digestate? Duckweed does not take much space to grow and can be used as fish and livestock feed.

www.duckweed49.com

Alfredo, there's one large plant in Switzerland /60'000 t/a) which treats the liquid fraction (from mechanical dewatering of digestate) along ultrafiltration and RO to produce a nutrient concentrate for agricultural use. Water fraction is free of C,N,P and can be discharged into natural water body. Technology works perfect but needs quite a bit of electricity for filtration.

Dear Mr Calt,

Thank for answering and the information. I found very interesting your paper and I look for the same kind of concept: To obtain subproducts from the water treatment, just as you show in your patent: http://www.google.com/patents/US8367372

I just have found your system very precious, but I cannot see is how clearly your system works. Seems to be a bit too complex and difficult to mannage such amount of specific subproducts. I look for something more practical for our latinamerican reality: to cope with nutrient extraction (for its utilization as fertilizer).

I also have a question: if MEF is a fermentation, its digestate should need to pass through an aerobic water treatment system in order to leave the water clean, is´nt it?

Best regards,

Alfredo Erlwein

Dear Louis Landesma,

Thank you for answering. Very interesting point.

Using aquatic plants have been a matter of my current research. I have worked with constructed wetland for a decade, but I have my doubts about its efectivity on this case:

Firstly, duckweeds as most aquatic plants are very good for biogas production. If we use the plants of the treatment back into the biodigestor, then all the nutrients go back to the diluted phase in the digestate, so we loose its treatment benefit. Same happens when using the activated sludge back to biodigestor.

To have a equilibrated balance of nutrients, the input/output ratio have to be 1, that means, we need to take out of the system the same amount of nutrients that will be comming into the system. The nutrients in the solid phase are easy to be out taken, but I wonder about the best system for cleaning that liquid waste water (from digestate).

Secondly, the amount of nutrients that aquatic plants uptake from wastewater are normaly very low because the nutrient demand from plants is very low in comparisson with the amounts of waste waters.

Thirdly. I am thinking of using constructed wetlans as part of the digestated system, but firstly I need to figure out how remove nutrients. One case is with membranes like FMX (Anti-Foulling Membrabe, similar to reverse osmosis ), but it needs high energy. Take a look on:

http://www.bkt21.com/digestate-treatment-and-nutrient-recovery/

https://www.youtube.com/watch?v=3ST2wUSt2QI

Best regards,

Alfredo

Dear Urs Baier,

Thanks very much for your answere. Yes, membrane treatments are excelent (see videos on RO up), but they are expensive and need a lot of energy. Our laws standards here in southamerica are unfortunatelly not so high and neigther are our financial capacities.

Kind regards,

Alfredo

You can use the duckweed grown on digestate in other ways - as poultry feed, cattle feed, fish feed etc.

This will help pay for treatment

Thank you Louis,

You are right, although it is another issue to include in the system.

Cheers,

Alfredo

I appreciate the good suggestions in relation to Alfredo's question. Answers by Edward and Louis in particular are very useful. However, given the concern expressed in Alfredo's question, need for low cost, low tech to avoid power outage, etc, using powdered activated carbon to adsorb the nutrients and drying the slurry would not only remove impurities for subsequent reuse of the treated wastewater, but the dried matter would readily make available the nutrients when applied as fertilizer. Water hyacinth could also be used as duckweed. However it also releases adsorbed material when it dies and remains in the water being treated.

Thank you Mr Adewumi,

Seems to be a right focus.

My big interest is nutrient recovery, over water quality, but they somehow go together.

Best regard,

Alfredo

Mr. Ertwein, the MEF process itself is very simple.  As long as the temperature and nutrient level is maintained the process works on a predictable cycle.  In production, there would be a battery of fermentation vessels maintained by a computer system to time the extractions and feeding to coincide with fermentation cycle. The subsequent separation of compounds is done using off-the-shelf separation processes currently in use in the chemical industry.  There is no need for pretreatment of the digestate.  One of the design changes we are considering is to take the post-separation fluid from our process and put it through a secondary fermentation that runs at a much higher pH to convert the volatile fatty acids into methane for sale or use within the process.  The current design estimates that the waste stream should be less than 5% of the inbound mass.  The water discharged should be usable for cooling tower, or agricultural application. 

Thank you Mr. Calt,

That was a good briefing to understand more MEF system. Sounds very interesting and useful when more products than biogas and nutrientes are expected to be obtained. Best regards.

Mr. Erwin, attached is a paper showing the compounds that can be produced from digestate and the potential economics involved.

Thank you Mr Calt. I already read it. Very interesting. Greatings from Chile.

Alfredo