Antibiotic resistance is one of the more serious public health problems of the century, and wastewater is one of the routes by which resistance genes move between environmental bacteria, clinical pathogens and the wider environment. WHO treats environmental AMR as a major component of the problem, because treatment plants sit at the junction between human use of antibiotics and the wider environment.
The monitoring problem is a practical one. Quantifying antibiotic resistance genes (ARGs) properly means qPCR or metagenomic sequencing, and neither is affordable at the frequency needed to see what a treatment plant is actually doing day to day. Suspended solids and ammonia, which can both be measured continuously, do not identify resistance genes. What they do is track the two things ARGs travel with and respond to: particulate material, and the biological process conditions that determine whether treatment is working.
Table of Contents
ARG Association with Particulate Matter
Mechanisms of Particle-Associated Transport
The reason suspended solids is a useful proxy is well established in the environmental literature: the majority of ARGs in wastewater and receiving waters are carried on particulate material rather than being truly dissolved.
- Bacteria: ARGs inside intact bacterial cells, the largest fraction, travel with the cells as particles
- Extracellular DNA associated with biomass: DNA bound to extracellular polymeric substances and floc travels with sludge and solids
- Adsorption to inorganic particles: ARGs adsorb to mineral and organic particulate surfaces, which is why settling removes them relatively effectively
- Truly dissolved fraction: the smallest fraction, and the hardest to remove by physical treatment
The practical consequence is that anything that improves solids capture — clarification, filtration, membrane separation — also reduces ARG transport, and solids carry-over into the effluent carries resistance genes with it. Particle size matters as well: finer particles carry more ARGs per unit mass and settle more slowly, which is why a plant can show good TSS removal and still pass ARG material through in the fine fraction.
ChiMay SS sensors measure suspended solids continuously by laser nephelometry over a 0–10,000 mg/L range, with accuracy of ±5% of reading or ±2 mg/L (whichever is greater) and a response time under 10 seconds for process control.
Suspended Solids as an ARG Proxy Parameter
Suspended solids does not measure ARGs, and it should never be presented as if it did. What it provides is a real-time indicator of the particulate transport pathway that dominates ARG movement, validated against the site’s own qPCR data. Where a site has established that relationship, SS trends show that solids carry-over is increasing well before a laboratory result confirms the ARG consequence.
The relationship has to be established locally. The ratio of ARG copies to suspended solids mass depends on the treatment train, the influent, the season and the sludge age, so a correlation derived at one plant does not transfer to another. The practical workflow is straightforward: run SS continuously, run qPCR periodically at the same points, verify that the two move together at that plant, and then use the continuous data for day-to-day control.
Ammonia Monitoring for Nitrification Assessment
ARG Impacts on Nitrogen Cycling
Nitrifiers are slow-growing organisms and are affected by antibiotics and other inhibitory compounds at concentrations that would be irrelevant to most heterotrophs. Ammonia-oxidising bacteria are more sensitive than nitrite-oxidisers, so the first sign of inhibition is usually rising effluent ammonia rather than rising nitrite.
Ammonia measurement therefore doubles as a proxy for the health of the biological treatment process. When nitrification is inhibited, effluent ammonia rises, and the same conditions tend to coincide with changes in the microbial community that also affect ARG composition. Online ammonia analysis is one of the cheapest way to detect process inhibition in real time.
ChiMay NH3-N sensors use ammonia electrode technology with accuracy of ±0.1 mg/L, temperature compensation across the process range, interference correction for potassium and sodium, and real-time output suitable for automated process control.
Ammonia and ARG Concentrations
The link between ammonia and ARG concentrations is indirect but operational: both respond to load and process performance. Reported field behaviour in municipal treatment plants shows a positive relationship between ammonia and ARG concentrations, with ammonia changes typically preceding measurable ARG changes by a few hours — the response time of the biological process.
The strength of the relationship varies by source:
- Pharmaceutical manufacturing effluent: strong coupling, because antibiotic load and ammonia load share the same source
- Hospital effluent: moderate coupling, dominated by human excretion and cleaning discharges
- Agricultural runoff: weak and variable coupling, driven by different sources, hydrology and dilution
Treatment Process Optimization
Real-Time Control Strategies
Solids-based control: trigger increased waste rate to reduce sludge age when secondary effluent solids rise, activate intermittent aeration, and alert the operator to a likely overload.
Ammonia-based control: extend mixed liquor aeration time when effluent NH3-N rises above the control band, reduce waste rate to hold a longer sludge age, and verify that nitrifier activity is intact with a respiration test.
Combined control is the normal configuration in a municipal plant. Representative setpoints:
- Secondary clarifier effluent SS: 10–15 mg/L
- Final effluent NH3-N: below 0.5 mg/L
- Aeration basin DO: 2.0–3.0 mg/L
- Sludge retention time: 10–15 days, adjusted with temperature
ARG Removal Through the Treatment Train
ARG removal follows solids removal closely, which is what makes solids monitoring useful for it. Typical removal progression reported in the literature:
| Treatment stage | SS removal | ARG removal |
|---|---|---|
| Primary clarification | 40–60% | Roughly a third to a half |
| Biological treatment | 70–85% | The majority of the total removal |
| Secondary clarification | 85–95% | Continues to improve with better solids capture |
| Tertiary filtration | 95–99% | Highest overall removal at the best-run plants |
The pattern is consistent across studies even where absolute numbers differ: biological treatment does most of the work, secondary clarification with good solids capture is where the plant either holds that removal or gives it back, and tertiary filtration determines the final effluent quality. Continuous SS monitoring at those three points tells an operator where they are losing it.
What a Sensor-Based Programme Looks Like
A workable programme at a municipal plant:
- Instrumentation: SS sensors on primary effluent, mixed liquor and final effluent; NH3-N analysers on aeration basin and final effluent; both integrated to SCADA
- Reference analysis: periodic qPCR for a defined marker set (for example sul1, tetM, intI1) at the same points, at a frequency the laboratory budget supports
- Correlation check: compare continuous SS and ammonia against the qPCR series quarterly and confirm the relationships still hold, including seasonally
- Control response: use SS carry-over alarms to trigger operational checks on clarification rather than waiting for the next laboratory result
The value of the programme is not that sensors detect ARGs; they do not. It is that they let a plant act within minutes on the process conditions that govern ARG release, while qPCR provides the periodic confirmation that the process is behaving as assumed.
Cost Perspective
Sensor-based screening reduces the number of molecular analyses required rather than replacing them: a plant that runs continuous SS and ammonia can sample for qPCR when the data indicate something changed, instead of on a fixed calendar. For the capital and operating cost of an SS/NH3-N station, the return comes from reduced laboratory load, earlier detection of nitrification inhibition, and chemical and energy savings from steadier process control. The specific figures depend entirely on plant size, existing instrumentation and laboratory cost structure — any universal payback estimate should be treated with suspicion.
Conclusion
Suspended solids and ammonia sensors do not measure antibiotic resistance genes, and any programme that implies otherwise is overstating what the instruments can do. What they measure is the particulate transport pathway that carries most ARGs and the nitrification performance that indicates biological treatment health. Used with periodic qPCR confirmation, they give a treatment plant continuous visibility of the processes that decide how much resistance material leaves in the effluent. ChiMay’s SS and NH3-N instruments provide that measurement layer.
