Aerobic mineralization is very similar to Aerated Compost Tea (ACT) or Actively Aerated Compost Tea (AACT)and  is one of the easiest ways to grow more plants using less fish in aquaponic or aquaculture systems. 

You may already have heard of ACT or ACCT being used in the soil systems thanks to the hard work of Dr. Elaine Ingham an American microbiologist and soil biology researcher and founder of Soil Foodweb Inc; an organization which has dedicated itself to restoring soil health.

Mineralization is a term used to describe the breakdown of organic solid waste into biologically available nutrients.  Mineralization is a form of nutrient cycling in aquaponics. However, instead of using compost to boost the microbial life and bio-available nutrients, we use the organic matter removed from the system.

The organic matter in our systems generally consists of fish waste, plant matter, old bacteria, fish feed, and other detritus that ends up in the water of the system. 

Terms to Know

Aerated Compost Tea (ACT) or (AACT)–  Water extracted compost made using finished compost; for ACT the dissolved oxygen must be greater than 6.0 mg/l.

Oxygen Reduction Potential (ORP)– The potential for a substance to become oxidized by oxygen. As DO increases the potential for substances to be oxidized is higher.

Dissolved oxygen (DO)– The amount of O2 dissolved freely into the water.

Turbidity– A measure of water clarity. The more suspended solids in the water the higher turbidity you will have. 

CFM– Cubic feet of air flow per minute. 

LPM– Liters of air flow per minute.

Anaerobic– Oxygen is not available In the water or chemically bound forms. Microbes tend to undergo respiration using other substances like fumaric acid, sulfate (SO4), and sulfur (S). This type of respiration is also called fermentation. This type of environment occurs at DO levels below 4.0 mg/l.

Aerobic– Oxygen is freely available, and microbes undergo respiration using oxygen; this type of environment  occurs at oxygen levels higher than 6.0 mg/l.

Anoxic– Lacking free O2 levels, but there is molecular oxygen bound to nitrate/nitrite; this type of environment occurs when DO oxygen levels at or near 0.0 mg/l.

Flocculate– Forming or causing the formation of small clumps or masses.

Facilitative microbes– Microbes that can live in more than one environment because they can use oxygen from multiple sources. For example, microbes that can survive in a combination of aerobic, anaerobic, and anoxic environments.

Obligate microbes– Microbes that can only live in a specific environment and are unable to change where they get their oxygen source. An example would be a strictly aerobic bacteria which can only survive in a DO higher than 6.0 mg/l.

Facultative anaerobe– Microbes that can survive in environments with a DO between 4.0-6.0 mg/l. 

Supernatant– Describes a liquid lying above solids which have settled out.

Heterotrophic bacteria– Bacteria that require a source of organic (living or once-living) carbon. These bacteria break down waste and are different from nitrifying bacteria which get their carbon from inorganic sources (CO2, carbonates, and bicarbonates).

History 

It is essential to remove the solids in both aquaponic and aquaculture systems. The build-up of organic matter contributes to the biological oxygen demand of the system, creating anaerobic zones that promote the development of pathogenic bacteria and hydrogen sulfide. 

Organic solids are collected using gravity or mechanical filters. These filters concentrate and remove the sludge from fish culture systems.

 In the past, disposal generally referred to the producers applying the removed waste directly on farmlands, drying it in geotextile bags, or paying for the removal of this nutrient-rich byproduct of aquaculture productions.

What early producers did not realize is that by disposing of the waste, they were throwing away thousands of dollars each year.

New research has shown that a large percentage of the nutrients excreted by fish are bound up in their fecal matter and are unavailable to the plants. That is where mineralization comes in.

Aerobic vs. Anaerobic

Mineralization is an easy and non-intensive process of breaking down solids. There are two methods; aerobic and anaerobic mineralization. In both methods, growers remove the solids from the system and put them into a tank separate from the system.

Aerobic Mineralization

In aerobic mineralization, air stones are added to the solids holding tank, which provides two functions.

First, they create agitation in the water, which keeps the organic solids suspended.  Second, they provide oxygen for the beneficial bacteria in the system.

Over time biofilms begin to form on the solids causing them to flocculate or stick together, and heterotrophic bacteria build-up in the tank.

 The heterotrophic bacteria consume organic waste and break it down, releasing the once-bound nutrients.

The process of aerobic mineralization depends on several water factors (pH, ORP, DO, Temperature, Nitrogen: Carbon ratio, SS). However, it takes roughly 18 days for 75% of the solids to be broken down, and 30 days for 100% solids breakdown.

 Once the solids have been broken down or have begun to break down, the air going into the mineralization tank can be shut off. Stopping air or agitation will allow the remaining SS to settle to the bottom*.

Once the solids have settled clear nutrient-rich water is left remaining, which is often called “supernatant.” Adding the supernatant back into the aquaponic system provides a boost of nutrients to the plants, including phosphorus, calcium, potassium, and various micronutrients that are otherwise unavailable.  

In anaerobic mineralization, decomposition from facultative or obligate anaerobic bacteria results in the breakdown of organic matter and denitrification. This process also results in the production of toxic gases such as methane, CO2, and hydrogen sulfide. 

Anaerobic Mineralization

During anaerobic mineralization, the solids are added to a air-tight tank where they sink to the bottom of the water column and allow for anaerobic bacteria to break down the organic solid waste. 

On a small scale an anaerobic digestor can be designed so that as you input waste, a similar volume is pushed out the other end. These designs are often called a plug and flow set up.

On a large scale solids can also be agitated using a propeller; this can also help to prevent anoxic zones. If using an agitator, it is crucial to limit the amount of oxygen introduced while mixing the water and solids.

Over time, the solids are broken down and released into the water column; like in aerobic mineralization, this nutrient-rich water is added back into the aquaponic system periodically. 

The added benefit is that rather than needing energy

1. Adding beneficial microbes such as Lactic Acid Bacteria (LAB) and Purple non-sulfur bacteria (PNSB), and yeast can aid in nutrient cycling and reduce potential for pathogens.

* Most of the SS precipitate out within 1 hour though often producers will wait for 12-24 before discharging the supernatant from the mineralization tank and adding it into their systems.  

** These microbes are generally recognized as safe (GRAS) by the FDA. These microbes have widely been used for wastewater treatment and in natural or regenerative farming practices. The microbes are heterotrophic, and many can live in both anaerobic and aerobic zones

Bottom line?

Mineralization is a natural process that can significantly reduce the amount of feed and other amendments in the system. 

Research has shown that the addition of supernatant can reduce the weight of feed needed to be fed per day to meet plant demands by 60-90%!

For those that use a Feeding Rate Ratio (FRR) also known as (Feeding Area Ratio) allowing for an adjusted feed rate of ~10-25 g/m2/day. For commercial producers, this means less start-up and operating costs and more profit.

The supernatant can be added monthly after full mineralization or to simplify the process, just add your supernatant every time you discharge solids to your system, whether that’s daily or weekly; you won’t regret it! 

Rules of thumb

• Size mineralization tanks to hold 28-30 days worth of solids discharged
• If the mineralization tank fills up in less than 28-30, it is no problem because most of the solids remain in the tank when the supernatant is discharged
• Aeration can be sized at 1-2 CFM/100 gallons (30-60 lpm / 400 L) with 4% adjustment for every 1000 feet of elevation gain above sea level
• Keep pH >= 6.8 
• Additional carbon sources can be added to boost production; best rates are still being researched 
• Every 3-6 months clean out the solids at the bottom of your mineralization tank. Overtime non digestible minerals build-up and can reach a level where they prevent heterotrophic bacteria from operating as efficiently

Additional Resources

Here is a link to the Agricultural Waste Management Field Handbook; this handbook was created for engineers and other curious folks.

Currently, researchers are working to create similar guides for the aquaculture industry; however, a lot of the information is spread amongst many publications.

I have included the handbook above because there is a lot that can be learned about how we use other livestock waste in agricultural systems and how we can implement them in aquaponics and aquaculture systems.