Survey of Suspended Solids in Irrigation Water of Ornamental Plant Nurseries and Effects of Filtration

Abstract

The objectives of this study are to (1) characterize the consistency of laser diffraction (LD) and automated dynamic image analysis (DIA) instruments in estimating the size of suspended peat particles in water and the sphericity of peat particles measured using DIA; (2) characterize the particle-size distribution of suspended solids in irrigation water sources from a survey of plant nurseries; and (3) analyze total suspended solids (TSS) upstream and downstream of fiber media and screen filters installed for filtration of recirculated water in commercial plant nurseries. Over 70% of peat particles had an estimated circularity value greater than 0.7, indicating a mix of elongated and spherical particles. The DIA and LD estimates of median particle diameter with circular particle-shape models yielded similar results when tested on three peat particle-size classes and two levels of TSS. TSS varied greatly in different water sources, with an average 3.3±0.4 mg/L mean±standard error, with a range of 2.5–4.5 mg/L for well water, average 4.7±1.2 mg/L (range of 1.6–9.9 mg/L) from uncovered catchment basins, and an average of 40.0±14.8 mg/L (range of 2.0–301.0 mg/L) from ebb-and-flood subirrigation return water, respectively. Across all water sources, TSS ranged from 1.6 to 301 mg/L, averaging 28±10.4 mg/L. The suspended-particle diameter in the 10th, 50th (or median), and 90th percentiles by total particle volume was 28, 116, and 347 μm, respectively, which is relevant when considering the amount of suspended solids that are likely to be removed by filters of different micrometer sizes. Fiber media and screen filters reduced TSS by an average 57.9±7.4% of the prefiltration TSS. Microscopy analysis of several fiber media filters showed that the pore sizes reported by vendors were smaller than the observed particle pore sizes. Multiple filtration stages would be ideal for ebb-and-flood water because of the high and variable TSS levels observed in recirculated ebb-and-flood water samples, the wide range of particle sizes and shapes, and the average removal of approximately half the TSS by a single stage of screen or fiber media filtration.