Black Soldier Flies & Recycling | Keeping Organic Leachates in Perspective : The Life and Times of BSF (Black Soldier Flies)
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Black Soldier Flies & Recycling | Keeping Organic Leachates in Perspective

by Terry Green on 10/22/13

Recycling organic leachates into soil is beneficial to the environment and cost saving as well. Leachates are a natural byproduct of organic wastes. Their chemical makeup and nutritional value in providing ecological viability to soil and their value in stimulating plant growth varies depending upon the source of the waste from which they are produced. This blog focuses on recycling leachate recovered from food scrap and agricultural wastes processed by Black Soldier fly (BSF) larvae.

Leachates from food scrap and agricultural wastes are made up of solutes, water soluble organic and inorganic compounds, left behind in the water phase which drains free of decaying waste. It forms as complex polysaccharides and carbohydrates, lipids, proteins, DNA, RNA, trace metals and minerals all undergo biodegradation. Metabolic wastes produced by microbes and insects feeding on the decaying organic waste also define its final chemical composition.

We all enjoy the benefits of leachates every day in our lives without a second thought. Figure 1 is an example of an instantly recognizable and familiar leachate.


Fig. 1. A common organic leachate prepared from tea leaves which following recovery of complex organic byproducts (see The Chemistry of Tea) has many beneficial attributes.

There is nothing fundamentally sinister, disastrous, necessarily toxic or even ecologically dangerous in using organic leachates applied in a beneficial manner to improve our quality of life, even the environment. This is certainly true regarding getting the most out of organic leachates recovered from decaying food scrap and agricultural wastes (see Black Soldier Fly Processed Food Scap | Foilant and Soil Applications, Amending Soil With Black Soldier Fly Processed Food Scrap Leachate, Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil and Is Composting an Earth Friendly Method of Recycling Plant and Food Scrap Waste?). For an interesting, very practical and humorous account on the many benefits that can be realized by recycling animal manure and urine generated on the farm readers ought to also consider reading Gene Logsdon’s book, “Holy Shit: Managing Manure to Save Mankind”.

Guidelines in most counties and municipalities around the world spell out the importance of keeping leachates (those formed from food scrap and other agricultural waste products such as animal manures spread out on farms and pastures) free from contaminating natural waterways, aquifers and wells. Excess runoff of leachates into natural waterways, for example, will promote algae blooms and increase microbial activity detrimental to aquatic life. Too much nitrate, a form of plant available nitrogen present in synthetic NPK fertilizers, animal manures and leachates allowed to permeate well water must likewise be avoided since nitrate accumulation in drinking water is hazardous to human and animal health since it oxidizes hemoglobin to methemoglobin, reducing the body’s capacity to deliver oxygen to tissues and organs.

The question of dealing with organic leachates efficiently and in an environmentally sound manner is especially pressing concerning recycling operations involving scaled up growth and harvesting of BSF larvae from food scrap and agricultural wastes.  There is great interest in identifying practical and economically sound industrial processes for scaling up production of BSF larvae grown off of biodegradable organic wastes (see for example, Black Soldier Fly Larvae | An Earth Friendly Feedstock?).

The average feed conversion ratio for BSF larvae grown off of food scrap waste based on pilot plant scale up studies measured at DipTerra LLC is ~ 3.0 (dry weight of food scrap feed stock: wet weight of larvae). Based on these findings, even a small commercially operating plant processing, for example, ~5 MT of food scrap per day (~1800 MT wet weight food scrap/year) and realizing a BSF larval harvest of roughly 0.33 MT per day (~120 MT wet weight larval/year) will begin to drown in leachate unless provisions are also in place for dealing with this byproduct.

In metric units, the amount of leachate generated while growing BSF larvae off of food scrap and agricultural wastes easily amounts to as much as  ~ 10% of the initial starting wet weight of the waste. Processing ~ 1800 MT (~1984 tons) of food scrap per year as in the above example would therefore result in the production of as much as 180,000 L of leachate per year (~48,000 gals). Growing BSF larvae off of biodegradable wastes on a large scale, in short, is not just a matter of dealing with efficient harvesting of the larvae, alone. Equally important is the issue of what to do with the leachate byproduct!

A very practical outlet for this volume of leachate is to use it as a soil amendment in farming operations akin to traditional recycling of animal manure familiar and long recognized for its value in improving soil fertility. Providing the same guidelines are followed as for any manure or NPK application in field operations, it is reasonable to anticipate that BSF leachate produced as a BSF byproduct in recycling food scrap may be yet another valuable soil amendment and potential income generator that is beneficial to farming operations, especially in helping jump start early growth and root formation of important agricultural crops.

Table 1 underscores this point. Nitrogen and potassium, important for plant growth and root formation, retained in the leachate fraction appear roughly comparable to that detected in animal manures.



It is evident from Table 1 that BSF leachate recovered from processed food scrap waste is not a fully balanced fertilizer. Phosphate is notably depleted in the leachate, possibly having been removed to some extent from the leachate by larvae growing off of the waste, but also possibly having fallen out of solution as a precipitate in the form of struvite, brushite and/or hydroxyapatite divalent alkali mineral complexes (see Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil ).

The absence of phosphate may not necessarily be a disadvantage since this also indicates that the leachate could however be used to add nitrogen and potassium in soils already having adequate phosphate levels, providing the option for delivery of nitrogen and potassium without overloading the soil with excess phosphate.

Since BSF leachate is nearly free of solids (see Table 1), it is also easy to disperse over farm land and pastures. Loading rates in dispersing leachate in incremental 1000 gal units per acre (equivalent to ~32 lb of total nitrogen per acre) for specific agronomic crops could be added to varying soil types for optimal management as is the case presently with liquid animal manures (see specific regional guidelines, for example, Estimating Manure Application Rates, and Fertilizing with Manure).

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