Nutrient-rich frass that is not yet a fertiliser

01 · The finding

The frass from KALARVO's fish recipes was rich in nitrogen and low in heavy metals. Untreated, it did not meet the hygiene requirement for a fertiliser, so it needs heat treatment before it can be used as one.

KALARVO tested three Kainuu fish side streams as feed for black soldier fly (BSF) larvae, over three rounds and 70 recipes. This article covers the second product of the process; the summary report covers the whole project.

02 · Measurements

What frass is and what we measured

Frass is what stays in the tray after the larvae are sieved out: their droppings mixed with uneaten feed. Samples were collected at the final harvest, kept frozen and analysed in an accredited laboratory, one composite per fish raw material. Heavy metals and hygiene were analysed from the lake-fish frass.

Frass from recipes with

Lake-fish side stream

Dry matter, %

77.6

Nitrogen, % of DM

4.0

N–P₂O₅–K₂O, % of DM

4.0–4.5–1.4
Rainbow trout filleting side stream

Dry matter, %

69.4

Nitrogen, % of DM

4.8

N–P₂O₅–K₂O, % of DM

4.8–2.1–1.0
Rainbow trout gut mass

Dry matter, %

63.4

Nitrogen, % of DM

4.6

N–P₂O₅–K₂O, % of DM

4.6–1.6–1.3

The recipes also contained potato, brewer's spent grain and, in most cases, wheat bran, so the figures describe the frass of the whole recipe.

03 · Nutrients

Rich in nitrogen, phosphorus from the bones

All three were nitrogen-rich, at 4.0–4.8 % of dry matter. That is above the roughly 3 % typical of commercial BSF frass, and it matches a protein-rich fish feed.

Phosphorus followed the bones. The lake-fish side stream, with its heads and bones, gave 2.0 % phosphorus, the filleting side stream 0.9 % and the gut mass 0.7 %. Bone phosphorus releases slowly: in the literature, only about a fifth of the phosphorus in meat-and-bone meal is as available in the first year as superphosphate, and it works best on acidic soils (pH below 6).

The C/N ratio is low (estimate: at most about 8–12), which points to fast-release nitrogen. Like any fresh, nitrogen-rich frass, it can harm plants through ammonium and salts, so maturity and conductivity are checked before use.

04 · Heavy metals

Low in heavy metals

In the lake-fish frass, every measured heavy metal was well below the Finnish limits for organic fertilisers: mercury about 15 % of its limit, cadmium about 12 %, and lead and arsenic under 1 %. Mercury was about half the level found in the larvae from the same trays, so in KALARVO the mercury risk sits with the larvae, not the frass: the mercury findings

05 · Hygiene

Not yet hygienic

Salmonella was not detected in a 25 g sample. E. coli, however, was above the top of the method's range (more than 2,400 MPN/g). After treatment, a frass fertiliser must contain no Salmonella and at most 1,000 E. coli per gram. Untreated frass is therefore not hygienic as such. That is the normal finding for raw frass, and it matches the literature.

The fix is known. The Finnish Food Authority's guidance requires the whole mass to reach at least 70 °C for 60 minutes in a closed, verifiable process at an approved facility, followed by a new test of the treated product. In the literature, that treatment has brought E. coli below 10 per gram.

06 · What this means

What this means

Frass from fish side streams has real fertiliser content and low metals. Once hygienised, it is a useful by-product, but not the revenue pillar: plan the business case without frass income, and give frass a proper treatment and sales route.

If you have a side stream you want evaluated, get in touch.

Kajaani University of Applied Sciences (KAMK) procured black soldier fly larvae rearing trials on Kainuu fish side streams as part of the KALARVO project (Processing fish side streams into value products). Manna Insect carried out the trials as the contractor, responsible for the trial design, measurements and analysis. Co-funded by the European Union.