Nutrient load is a key term in water and wastewater treatment and describes the amount of dissolved nutrients, in particular nitrogen (N) and phosphorus (P), that are transported in a water cycle or wastewater system. This parameter is of particular importance as nutrients are essential for biological processes in wastewater treatment plants, but excessive concentrations can also lead to environmental problems such as eutrophication and water quality impairments.
The precise determination and control of the nutrient load is essential for the design and operation of water treatment plants. It influences the efficiency of biological treatment systems, compliance with legal limits and the sustainability of water cycles.
The nutrient load is defined as the total amount of a specific nutrient that is transported through a system in a defined period of time. It is typically expressed in kilograms per day (kg/d).
Table of contents
Importance of the nutrient load in practice
The nutrient load is a critical parameter in various areas of industrial water and wastewater treatment:
1. biological wastewater treatment
- Nutrients such as nitrogen and phosphorus are essential for the growth and activity of microorganisms that break down pollutants in biological systems (e.g. activated sludge).
- A balanced nutrient ratio (C:N:P) is crucial for stable biological purification.
- Optimum ratio: 100:5:1 (carbon:nitrogen:phosphorus).
- Carbon (C) is represented by the COD.
2. eutrophication and environmental protection
- Excess nutrients in bodies of water lead to eutrophication, an excessive growth of algae and microorganisms.
- This process removes oxygen from the water and endangers aquatic ecosystems.
- Legal limits for nutrient concentrations (e.g. total nitrogen and total phosphorus) in wastewater must be complied with.
3. process water treatment
- In closed water circuits, such as in the food or chemical industry, an excessive nutrient load can lead to biofouling, which impairs the efficiency of systems such as heat exchangers or reverse osmosis systems.
Types of nutrients and their importance
The nutrient load comprises several chemical forms that have specific properties and effects:
1. nitrogen (N)
Ammonium (NH₄⁺):
- Main form in anaerobic and low-oxygen systems.
- May be toxic to aquatic organisms if the concentration is too high.
Nitrate (NO₃-):
- Oxidized form formed in nitrifying systems.
- Mainly responsible for eutrophication in water bodies.
Organic nitrogen:
- Bound in proteins and amino acids.
- Must be converted into ammonium by hydrolytic processes.
2. phosphorus (P)
Orthophosphate (PO₄³-):
- Biologically available form that can be used directly by microorganisms.
- Important for cell structure and energy supply (ATP).
Particulate phosphorus:
- Form bound in particles that can be removed by chemical or physical processes.
Technologies for controlling and reducing the nutrient load
Controlling the nutrient load requires specific technologies that are tailored to the respective requirements and challenges.
1. nitrogen removal
Nitrification and denitrification:
Nitrification:
- Oxidation of ammonium to nitrite (NO₂-) and further to nitrate (NO₃-) by nitrifying bacteria (e.g. Nitrosomonas and Nitrobacter).
- Requires aerobic conditions (oxygen supply).
Denitrification:
- Reduction of nitrate to gaseous nitrogen (N₂) under anoxic conditions.
- carbon source (e.g. methanol or acetic acid) is required.
Ammonia stripping:
- Removal of ammonium by blowing out with air or steam at an increased pH value.
Photo: Post-treatment of wastewater from our biogas plant with biomass recirculation, nitrification and denitrification
2. phosphorus removal
- Addition of precipitants such as iron(III) chloride, aluminum sulfate or lime.
- Binding of phosphorus to insoluble salts (e.g. iron phosphate), which can be sedimented or filtered.
Biological phosphorus removal:
- Utilization of polyphosphate-accumulating organisms (PAOs) that absorb phosphorus into their cells.
- Cyclical change between aerobic and anaerobic conditions required.
3. membrane-based processes
- Removal of dissolved nutrients by physical separation.
Nanofiltration (NF):
- Selective removal of divalent ions such as phosphate.
4. biofiltration
- Use of bioactive filters that reduce nutrients through biological processes and adsorption.
- Ideal for the polishing stage after biological treatment.
Photo: Our ALMA BioFil Compact biofiltration system with nitrification and denitrification
Challenges in controlling the nutrient load
Fluctuating loads:
- Production peaks and cleaning cycles lead to fluctuations in the nutrient load.
- Solution: Use of buffer and mixing tanks.
Nutrient imbalance:
- Uneven distribution of carbon, nitrogen and phosphorus can impair biodegradation.
- Solution: Dosing of external carbon sources or chemical precipitation.
Costs:
- Precipitants and energy requirements for aeration increase operating costs.
- Solution: Integration of energy-efficient technologies such as anaerobic treatment.
Conclusion
The nutrient load is a key parameter for the planning, control and optimization of water and wastewater treatment plants. Its control is crucial for complying with legal requirements, stabilizing biological processes and minimizing environmental impact. Modern technologies such as biofiltration, membrane processes and chemical precipitation offer effective solutions for the reduction of nitrogen and phosphorus. A sound understanding of the nutrient load not only enables the efficient operation of wastewater treatment plants, but also contributes to sustainability and resource conservation.
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