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However, statistical analyses indicated that there is ameaningful difference between the rates of sludge reduc-tion in the reactors with the blanks in two runs (p>0.05).Due to similar medium conditions of the reactors andthe blanks, this meaningful difference highly confirmedthe effectiveness of the worms in the reactors for sludgereduction.DiscussionIn the present study, the rate of sludge reduction in thefirst and second runs of the reactors were 32% and 33%and in the runs (1) and (2) of blanks were 16% and 12%,respectively. In the reactor containing the aquatic wormsof Lumbriculus variegatus, which were experimented byHendrickx et al., the rate of sludge reduction was almostthree times higher than in the blank (without worms)[10]. In the present study, the sludge reduction rate inthe worm reactors were 2 to 2.75 times higher than inthe blank. This difference may be based on the differ-ences between the methods of experiments in tworesearches. In another survey in China, sludge reductionwas studied by using of a combination of protozoa andmetazoa and the rate of reduction (in MLSS) was from45% to 58%, compared to that of the blank. In this sur-vey, the DO concentration and hydraulic retention timeof sludge were considered as 1–4 mg/L and 6–13 hours,respectively [20]. The higher efficiency of sludge reduc-tion in this study may be resulted from the use of acombination of protozoa and metazoa in one reactor.In another study, carried out in China on the aquaticworm species of Tubificide, the average TSS reduction inthe sludge was 48% and the DO concentration was ≥2mg/L [13]. A comparison of the 48% reduction of the above-mentioned research with that of the present re-search (33%) shows that the aquatic worm Tubificidehas a better performance than the species of Lumbricu-lus variegatus. It may be noted that, Wei and Liualternately used two reactors as series containing worms.In another research which was performed in theNetherlands in an aerobic reactor on the activatedsludge containing worm species of Lumbriculus varie-gates [14], almost similar result for sludge reduction(up to 30%) compared to our study (33%), wasobtained, which is because of similar worm species andtreatment method (activated sludge). In China, anothersurvey was carried out by Guo et al., [21] in which, areactor containing worm species of Tubificide wasused to reduce sludge and an efficiency of 46.4% wasobtained. The SRT (Solid Retention Time), HRT (Hy-draulic Retention Time) and DO concentration in thisresearch were 30 days, 15.4 hours, and 0.5-3 mg/L, re-spectively [21]. Better results in this study may berelated to longer SRT (4 times more) and type of theworms (Tubificide).Wei et al, showed that the performance of the wormspecies Oligochaeta in the conventional activated sludge(CAS) system could lead to considerable reduction insludge and improve its settling characteristics [22]. Thesimilar results of sludge reduction was obtained in ourstudy on Ahvaz WWTP with CAS process.The daily rate of sludge reduction by four species ofworms was studied by Liang et al [9]. These rates were0.1 to 0.54 mg sludge/mg worm, based on the order orbody size of the worm [9]. The average sludge reductionin our study (0.45 mg sludge/mg worm per day) is al-most near to the maximum rate of sludge reduction inthe above-mentioned research. The quantitative results of this research, regarding thesludge reduction by worm species Lumbriculus variega-tus, were in agreement with Hendrickx's study (36%) [23].But, no meaningful relationship was found in our re-search between DO concentration and the rate of sludgereduction. This disagreement may be arising from differ-ences in worm habitats and their living conditions, thetype of reactors and operational conditions.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsYB: conception and design, generation of data, collection of data, assemblyof data, analysis of data, interpretation of data, drafting of the manuscript.MF: conception and design, interpretation of data, drafting of themanuscript, revision of the manuscript, approval of the manuscript. NJ:interpretation of data, drafting of the manuscript, revision of the manuscript,approval of the manuscript. TH: conception and design, revision of themanuscript, approval of the manuscript.