Why Is Stack Odor Removal in Feed Mills So Challenging?

Introduction

Feed mills are an indispensable part of Turkey's agricultural and livestock sector. Yet these facilities operate at the center of an extremely intense and persistent odor problem — one that is inherent to their production processes. Complaints from neighboring residential areas, risks during environmental inspections, and concerns over worker health force feed mill managers to confront this issue head-on.

So why is stack odor in feed mills so difficult to eliminate? The answer lies in the production chemistry itself.

Rendering Odor: The Root of the Problem

The most critical odor source in feed production is the rendering process. The compounds released during the high-temperature processing of animal raw materials (bone, meat trimmings, offal, blood meal, etc.) form an extremely complex chemical mixture:

Sulfur-based compounds: Hydrogen sulfide (H₂S), methyl mercaptan, dimethyl sulfide
Nitrogen-based compounds: Ammonia, trimethylamine, indole, skatole
Fatty acids: Butyric acid, propionic acid, isovaleric acid
Aldehydes and ketones: Acetaldehyde, hexanal

Each of these compounds has a different chemical structure, a different odor threshold, and a different reaction mechanism. Removing all of them with a single method is technically extremely challenging.

5 Core Reasons Why Stack Odor Removal Is So Difficult

Multi-Component Odor Mixture

In a standard industrial odor removal application, the target is typically a single pollutant — for example, only H₂S or only ammonia. In feed mills, however, the gas leaving the stack is a mixture of dozens of different compounds. This makes it impossible for any single chemical or single technology to succeed on its own.

Variable Process Load

Production processes in feed mills do not run at a constant rate. As raw materials change, formulations change, and extruder or pelletizer loads shift, the intensity and composition of stack odor changes significantly as well. This variability causes systems operating at fixed dosage to fall short.

High Moisture and Temperature

Rendering and drying stacks produce hot gases carrying high moisture loads. These conditions significantly reduce the efficiency of certain odor removal technologies — activated carbon filters in particular. Moisture reduces adsorption capacity, while high temperatures can damage certain biological filter media.

The Difficulty of Meeting the Legal Limit

In Turkey, the legal odor emission limit for industrial stacks is set at 1,000 OUE/m³ (odor units per cubic meter). Pre-treatment measurements at feed mills typically fall in the range of 3,000 – 8,000 OUE/m³ — and in some processes, values climb well beyond this. Bringing such a high starting value down below the legal threshold requires the system to operate at both very high capacity and very high efficiency.

Multiple Stack Points

In feed mills, odor does not originate from a single source. The extruder stack, dryer stack, cooler exhaust outlet, raw material storage areas, and wastewater ponds each produce odor at different intensities and with different compositions. Bringing all of these points under control simultaneously requires comprehensive system design.

How Does Ozone Address These Challenges?

Ozone (O₃), through its powerful oxidizing properties, chemically breaks down odor compounds. It converts sulfur-based compounds into sulfate and nitrogen-based compounds into nitrate — leaving no harmful residue in the process.

Two ozone application methods are commonly used for feed mill stacks:

Scrubber + Ozone Combination: Stack gas is first passed through a wet scrubber, after which ozone is applied to the scrubber water or the outlet gas. This approach is particularly effective for moisture-laden gases.
Direct Gas-Phase Ozonation: Ozone gas is injected directly at the stack outlet. This method is preferred for processes with low moisture content.

At OCS Ozon, we analyze the odor profile of each facility individually, design a site-specific system, and commit to bringing results below the legal limit of 1,000 OUE/m³.

Conclusion

Stack odor removal in feed mills is an exceptionally demanding engineering challenge — driven by a multi-component chemical composition, variable process loads, high moisture and temperature conditions, and strict legal standards. Overcoming this challenge requires not only the right technology, but also the right design for each specific facility.