Ignoring the solid phase significantly underestimates estrogenic steroid hormones in dairy shed effluent and effluent treatment systems.

Free biologically active estrogenic steroid hormones (ESHs) 17α-estradiol, 17β-estradiol, and estrone were quantified in liquid and solid phases of a covered anaerobic pond (CAP) dairy effluent treatment system (ETS). Total mean concentrations (dissolved and particulate) of ESHs in CAP influent, sludge, and effluent were 4171 ng/L, 93,601 ng/L, and 4346 ng/L, respectively. Peak ∑ESH concentrations occurred in dairy shed effluent during April and July, coinciding with late pregnancy and calving periods. Organic carbon normalised adsorption coefficients (Koc) ranged from 3.06 to 3.78 mL/g, consistent with published soil and wastewater sludge values. Up to 99% of the total mass of ESHs in the ETS samples was detected in the particulate phase, underscoring the need to consider this fraction in dairy effluent treatment systems. 17α-estradiol dominated total ESHs in influent (2869 ng/L), while estrone predominated in samples of the CAP sludge (85,414 ng/L) and effluent (3140 ng/L). Anaerobic treatment reduced the total concentrations of 17α- and 17β-estradiol but increased the total concentration of estrone. Estrogenic equivalents were dominated by 17β-estradiol and were preserved or elevated post-treatment. The CAP system exhibited highly variable and largely ineffective removal of ESHs and estrogenic activity. Findings highlight the need to optimise CAP operation to improve energy recovery while mitigating environmental risks from ESHs in treated effluent.