CEA recovered solution — separate sanitation, treatment and reuse boundaries

Matching EC does not make recovered solution safe

The aeroponics guide separated spray volume from root-chamber recovery. Before returning recovered solution to a tank, record EC, pH and temperature separately from turbidity, deposits, biofilm, microbial tests and treatment residuals.

Recirculation sends the same liquid through multiple roots and pipes, so a problem can travel through shared paths. UVM Extension describes cleaning and sanitizing food- and water-contact surfaces as critical controls. Virginia Cooperative Extension's food-safety guide also describes water's role in irrigation, nutrient delivery, cleaning and sanitizing.

Do not collapse four states into “water quality”

An in-range EC with no treatment record is not “ready for reuse.” Treatment also does not automatically restore nutrient composition or crop compatibility.

Record treatment purpose and side effects together

Filtration can reduce particles without removing all dissolved compounds or microbes. UV, heat and oxidants depend on turbidity, flow, contact time, temperature, dose and rinsing. Save the actual conditions rather than one method label.

For chemical treatment, verify compatibility, residuals, by-products and effects on crops, workers and discharge. A published study reported chlorate detection after sodium-hypochlorite sanitation when systems were not rinsed. The PubMed study is an example of why treatment conditions and residual checks cannot be separated. Do not transfer its dose or contact time to an installation.

Turn a synthetic log into a reuse classification

The following function returns REJECT, TEST, HOLD or REUSE_CANDIDATE. It is not a sanitation test; it only prevents missing records or absent treatment evidence from silently becoming a reuse command.

from math import isfinite

def classify(log):
    required = ("ec", "temperature", "turbidity", "sanitized",
                "residual_checked", "sample_age_h")
    if any(key not in log for key in required):
        return "REJECT"
    numeric = (log["ec"], log["temperature"], log["turbidity"], log["sample_age_h"])
    if not all(isfinite(x) for x in numeric) or log["sample_age_h"] < 0:
        return "REJECT"
    if log["turbidity"] > 2.0:
        return "HOLD"
    if not log["sanitized"] or not log["residual_checked"]:
        return "TEST"
    if log["sample_age_h"] > 2:
        return "HOLD"
    return "REUSE_CANDIDATE"

cases = [
    {"ec": 1.5, "temperature": 22, "turbidity": 0.8,
     "sanitized": True, "residual_checked": True, "sample_age_h": 1},
    {"ec": 1.5, "temperature": 22, "turbidity": 0.8,
     "sanitized": False, "residual_checked": False, "sample_age_h": 1},
    {"ec": 1.5, "temperature": 22, "turbidity": 3.0,
     "sanitized": True, "residual_checked": True, "sample_age_h": 1},
]
print([classify(case) for case in cases])

The output is ['REUSE_CANDIDATE', 'TEST', 'HOLD']. REUSE_CANDIDATE is not an approved return. Reuse still requires the required tests and the site's operating procedure to authorize the destination loop.

Check the system boundary before returning liquid

Limit the destination when returning from an isolation container, and record operation ID and liquid lot. When an anomaly occurs in one root chamber, define whether the shared tank, return line, filters and nozzle rows form one exposure boundary. Do not move questionable liquid to another zone merely because EC matches.

Scope

This guide covers classification, treatment records and pre-reuse boundaries. Pathogen-specific assays, chemical doses, food-law release decisions and treatment-equipment guarantees remain outside scope. Next is connecting nutrient chemistry and sanitation as two ledgers for reuse.