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In a storm event, a first flush (FF) phenomenon occurs when most of the pollution load is transported in the initial part of the event discharged volume. The objectives of the study were to consider its severity for a number of pollutants as well as to recognize rainfall–runoff characteristics that influence it. The study was conducted in a semi-arid region of a mixed residential and commercial urban catchment in Iran. Ten major rainfall events were monitored for total solids (TS), total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), Zn, and Pb. Total suspended solids and COD were usually above the permitted discharge. Power and polynomial functions fit well to the normalized curve of the distribution of pollutant load with volume. Discharge loads of TN, Pb, and Zn were approximately uniform. There was a relatively weak FF for TS, TSS, and COD. No correlation between FF load of TS and COD and rainfall–runoff characteristics was observed, but the amount of the FF load of TSS increases when the intensity and duration of a rainfall event increase. Key words: first flush, urban runoff quality, pollution load distribution, event mean concentration, urban stormwater runoff.

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The ever-increasing number and production capacity of petroleum refineries in recent years have intensified the need for developing an effective and practical method for treating their wastewaters. In this study, the application of Fenton process with scrap iron powder was investigated for the treatment of a bio-refractory petroleum refinery effluent. Response surface methodology was employed with a cubic IV optimal design to optimize the process using chemical oxygen demand (COD) removal as the target response. H 2 O 2 /COD, and H 2 O 2 /Fe mass ratios as well as pH were considered as the relevant parameters. A COD removal of more than 83 % was achieved under optimal conditions (H 2 O 2 /COD 10.03, H 2 O 2 /Fe 2.66 and pH 3.0) within 90 min. Kinetics studies were conducted to investigate the effect of reaction time on COD removal. In addition, the role of post-coagulation on COD removal under optimal conditions was investigated and it was found that 37 % of COD removal occurred due to coagulation, indicating its high potential in the Fenton process.

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