Health indicators of biological systems in wastewater treatment: concept of biological performance (BR) vs. biological degradation rate (BDR).
Biological degradation rate of organic matter (B.D.R.)
Definition
TDB is the rate at which biomass consumes and removes biodegradable soluble organic matter in the biological reactor.
Key concept on COD
COD can enter a biological system as suspended COD or dissolved COD. Soluble COD is further divided into biologically degradable COD and recalcitrant or hardly biodegradable COD. The yield must be calculated on the soluble COD, but knowing the biodegradability nature of the COD in the system is necessary to correctly interpret the analytical results.
B.D.T. as an indicator of the health of the biological system.
Biological yield (B.Y.)
Definition
Biological yield is the amount of biomass (sludge) generated by the biological process for each unit of organic matter removed. In other words, it defines the conversion rate of organic matter to biomass.
Biological performance as an indicator of health
A healthy system, with good nutritional balance and a well-structured ecosystem, will have high yields.
- Normal biological yield (healthy): 0.4 – 0.6
- Stressed, old or inhibited system: < 0,3
Factors affecting health indicators of biological reactors (T.D.B. and R.B.)
Operational factors
- Dissolved oxygen
- Temperature
- Nutrient adjustment
- pH and alkalinity
- Mixing strategy / reactor type
- F/M Ratio
Water factors
- Soluble biodegradable fraction
- Nature of the industry
- Inhibitory compounds
- C/N ratio
- Age of mud
Demineralization of primary treatment: concept
A typical primary treatment includes:
- Coagulation
- pH correction
- Flocculation
Demineralizing primary treatment means eliminating the contribution of elements of mineral origin, especially when traditional metallic salts are used as:
- Aluminum polychloride (PAC)
- Alumina sulfate
- Ferric chloride
Demineralization consists of replacing these salts with more sustainable organic-based coagulants.
It also includes reducing or eliminating the use of alkalinizers such as soda or CaO.
Effects of primary treatment demineralization on health indicators.
Operational level
TDS reduction
Chlorides / Sulfates
Eliminating metallic coagulants significantly reduces:
- Chlorides (very corrosive → risk of pitting)
- Sulfates
This improves the water’s suitability for:
- Industrial reuse
- Agricultural use
If water passes through membranes, the absence of Fe/Al reduces OPEX significantly.
Sludge quality
- Al/Fe content: Although there are no specific regulations, high concentrations may damage sensitive crops.
- Electrical conductivity (EC):
- CE < 2 dS/m → suitable for most crops.
- 2-4 dS/m → risk for sensitive crops.
- 4 dS/m → high risk of salinization.
Other benefits
- Increased biomethanization potential
- Higher organic matter content → better for compost or agricultural use.
Biological level
Typical toxicities
This article delves into the toxic effects affecting TDB and R.B.
The most problematic species include:
- Quaternary ammoniums
Used as disinfectants; inhibit microbial metabolism. - Heavy metals and aluminum.
Metals such as Hg, Pb or Cd are highly toxic.
The article focuses especially on Al³⁺ toxicity, which causes:- Growth inhibition
- Enzymatic damage
- Loss of membrane integrity
- Oxidative stress (formation of ROS)
- Volatile aromatic compounds (VOCs).
Benzene, toluene, etc., affect microbial activity.
Effects on biological performance (B.R.)
When the system is inhibited:
- Bacteria expend energy to survive
- Increases maintenance respiration
- There is more cell lysis
- Part of the COD is oxidized without generating biomass.
- Less biomass is generated
When the inhibition disappears:
- R.B. back to >0.3 (normal 0.4-0.6)
- Efficient metabolism
- Stable growth
- Fewer episodes of bulking/foaming
- Better decantability
- Reduced biomass loss
Effects on the rate of biological degradation (T.D.B.)
With inhibition:
- Reduced microbial activity
- Maintenance is prioritized over growth
- Lower soluble COD consumption by time
- Risk of overload
When the inhibition is removed:
- Enzymatic reactivation → TDB increase.
- Well-formed flocculus
- Low mortality
- Reduction of metabolic stress
- Recovery of nitrification
Economic level
Demineralizing primary treatment brings clear OPEX advantages:
- Reduction of chemical consumption:
- Less coagulant
- No alkalinizer
- Less flocculant
- Optimization of oxygen consumption
- Optimization of nutrient use
- Sludge reduction → lower management costs
- Reduced risk of sanctions
- Fewer unscheduled stops (bulking/foaming)
- Increased biomethane yield
- Organic products are non-corrosive → longer service life of the installation.
