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AbstractSludge production is an avoidable problem arising from the treatment of wastewater. The sludge remained aftermunicipal wastewater treatment contains considerable amounts of various contaminants and if is not properlyhandled and disposed, it may produce extensive health hazards. Application of aquatic worm is an approach todecrease the amount of biological waste sludge produced in wastewater treatment plants. In the present researchreduction of the amount of waste sludge from Ahvaz wastewater treatment plant was studied with the aquaticworm Lumbriculus variegatus in a reactor concept. The sludge reduction in the reactor with worm was compared tosludge reduction in a blank reactor (without worm). The effects of changes in dissolved oxygen (DO) concentrationup to 3 mg/L (run 1) and up to 6 mg/L (run 2) were studied in the worm and blank reactors. 36254
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No meaningfulrelationship was found between DO concentration and the rate of total suspended solids reduction. The averagesludge reductions were obtained as 32% (run 1) and 33% (run 2) in worm reactor and 16% (run 1) and 12% (run 2)in the blank reactor. These results showed that the worm reactors may reduce the waste sludge between 2 and2.75 times higher than in the blank conditions. The obtained results showed that the worm reactor has a highpotential for use in large-scale sludge processing.Keywords: Sludge reduction, Aquatic worms, Ahvaz wastewater treatment plant, Lumbriculus variegatus IntroductionBiological treatment is the most used technology in was-tewater purification. Generation of large amounts ofsludge is one the major features undertaken in the somebiological wastewater treatment plants [1]. Such excesssludge has to be properly treated prior to final disposal,even though the cost of sludge treatment is extremelyhigh, accounting for up to 60% of the total operating costin a wastewater treatment plant [2].At the present time, there are not any principle activ-ities for pollution control of disposal sludge from muni-cipal wastewater treatment plants (WWTPs) in Iran, andtherefore it is generally unstable [3].
Reuse or dischar-ging of raw sludge would lead to many hazardous mate-rials which would pollute natural resources such aswater, soil and agricultural products [4]. Activated sludgeprocess is one of the most comprehensive and widely-used biological processes in domestic and industrial treatment plants in Iran [5]. This process is generated oforganic and inorganic sludge to a great deal, that is con-sidered a disadvantage [6]. Based on the results of thestudy that was done by Farzadkia in four local municipalwastewater treatment plants in Tehran which workedbased on activated sludge process, except one of them,the others disposed untreated sludge [7].Strict environmental regulations regarding wastesludge in wastewater treatment plants and the high ex-penditure of treatment, transportation and disposal ofsludge have intensively led to the sludge minimization[5]. The important methods for the reduction of excesssludge are: endogenous metabolism, uncoupling metab-olism, increase of dissolved oxygen in the reactor, oxicsettling-anaerobic, ultrasonic cell disintegration, alkalineheat treatment, and growth of controllable predators.Also oxidation of a part of produced sludge is done byoxidizing materials such as chlorine and ozone [2,8].Recently, the use of protozoa and metazoa has beenproposed as a biological method for sludge reduction.This method, which is based on relationships present inthe food chain and causes the overall reduction of the biomass, has gained more attention due to its low energyconsumption and the lack of subsequent pollution [9]. Anumber of researches have been carried out on the re-duction of disposed sludge by aquatic worms. Hendrickxet al. compared the performance of a species of aquaticworms called Lumbriculus varigatus in a reactor withperforated media containing worms with a usual reactorwithout any worms.
The results of this research showedthat the rate of total suspended solids (TSS) reduction inthe reactor containing worms, in most of the cases, wasthree times greater than the reactor without worms [10].In addition, Huang et al. studied the reduction of thesludge produced by activated sludge process by a speciesof aquatic worms called Tubifex tubifex in a reactor [11].An ecological method regarding the use of four types ofmicrofauna for the reduction of waste sludge has beensurveyed by Liang et al. The results of this surveyshowed that the rate of sludge reduction depends on theclassification and body size of the microfauna, and thespecies in the kingdom of Oligochaeta [9]. Wei et al.found out that worms, according to body size, are thebiggest organisms in the sludge treatment cycle, com-pared with protozoa. They are easier to maintain anddue to body size, have enough capability in sludge reduc-tion [12]. Wei and Liu designed a combined reactor forthe reduction of sludge using both free-flowing wormsand sessile ones. In this experimental reactor, the TSS ofsludge was reduced by 48 percent, which is mainly dueto the presence of Tubificidea [13]. Rastak studied thepossibility of reducing activated sludge in wastewatertreatment plants using aquatic worms of Oligochaeta atexperimental scales, and the environmental factorsaffecting the performance of worms were surveyed. Thisresearch confirmed the use of worms as a protein-supplysource in the food for fish and domestic animals [14].Although the advantages of aquatic worm applicationfor sludge reduction are known, this method has not yetbeen studied in any scale in Iran. The objectives of thisstudy were identification of appropriate and endemic spe-cies of aquatic worm called Lambriculus variegates,adap-tation of life natural environment of worms to laboratoryconditions, determination of sludge reduction by Lambri-culus variegates worm in Ahvaz Wastewater TreatmentPlant (WWTP) and determination of effective DO con-centration in worm reactor. Ahvaz WWTP operatedbased on activated sludge process for wastewater treat-ment and has two stages of anaerobic digester for sludgestabilization. The first stage of anaerobic digester wasdamaged and was out of service at the time of this study.Comparison of the microbial quality of disposal sludge inAhvaz WWTP and USEPA criteria showed that thesludge was not in class A or B conditions. Hence, itshould not be disposed to the environment or reused forany purpose [15].Materials and MethodsProvision of the worms from the specified species was achief concern in this research. Due to the possibility ofaccess to this species in areas with organic and decom-posed materials, the probable homes of this species wereidentified. Samples were taken from the benthic materi-als of Khuzestan water reservoirs including KaroonRiver, Maleh Stream, Shadegan Wetland, and Dez riverby using grab model of Van Vee [10]. Microscopic pic-tures of the samples were then taken and identified with“Freshwater Biology” [16]. No worms belonging to thesubclass Lumbiculidae were observed.Due to the study carried out on the spring pools ofKermanshah province in the west part of Iran [17], a fewsamples were taken and observed in this area. Finally,the species of Lumbriculus variegates from the Oligo-chaeta subclass and Lumbiculidae family were found inthe sediments deposited of Jabery and Ravansar springs.To maintain the environment of the worms collected aswell as their compatibility with the new environment,the vessels containing worms were aerated, and thetemperature of the medium was constantly controlled[18]. In order to take samples of aquatic worm speciesand to carry out field tours, a collection of outdoorequipment including the Sampler VN, a GPS, electricalconductivity meter, mercury and digital thermometers,kit of DO analyzer, a sieve with one millimeter mesh,and other equipments for transportation of sampleswere used [14].The worm reactor consisted of a beaker containingboth waste sludge and worms. It contained 1500 mg wetweight of the aquatic worm and 100 mL of waste sludgeof Ahvaz WWTP. The blank reactor was investigatedunder the same conditions but without any worm. Thewaste sludge was daily provided from the return line ofChoneibeh WWTP in the west of Ahvaz city. Experi-ments were carried out in two parallel runs with two dif-ferent dissolved oxygen contents. Each run consisted of8 batch steps and took 1 day. Therefore, each run took 8days; repetition of each batch step was done 8 times.The concentration of the dissolved oxygen in steps oneand two were kept up to 3 and 6 mg/L, respectively [10].pH, temperature and TSS of the incoming and outgoingsludge were determined every 24 hours at the end ofeach step.Every 24 hours, aeration was stopped for a short timeand, by using a fine mesh; the worms were separatedand prepared for entering to the next batch step [14].The species of the aquatic worms during the days of theexperiment were similar: they were selected among thesame initial population under the experiment [10]. Allexperiments were carried out by using the standardmethods [19], and the results were assessed based onANOVA analysis and T statistical test in order to investigate the average differences at a reliable level of95%. The Smirnov-Kolmogrove test was used to deter-mine the normal distribution of the findings. The effectof water evaporation in aeration was noticed via addingsample volume about 20% in the start time. Selection of20% was based on the pretests.ResultsTSS in the incoming and outgoing sludge of the reactorsand blanks are indicated in Figure 1 and Figure 2, whichshow that, during the whole days of the two runs, TSSin outgoing sludge was less than that the incomingsludge. The minimum and maximum amounts of TSS ofsludge entry the reactors and blanks were 2456 and7184 mg/L, respectively. The efficiency of sludge reduc-tion in the reactors, as well as the blanks are shown onFigure 3, which indicates that the efficiency of sludge re-duction in the second-phase reactor was 33%, slightlyhigher than 32% of the first-phase reactor. The rates ofsludge reduction in the blank reactors in the two phaseswere 16% and 12%, respectively. The average sludge re-duction by the species Lumbriculus variegatus wasfound to be about 0.45 mg of sludge per one mg of theworm per day. The Smirinove- Kolmogrove test on thesuspended solids in the sludge and the dissolved oxygenshowed that the data followed a normal distribution.Based on the data obtained from runs (1) and (2) in thereactors, no significant difference was observed betweenthe sludge reductions in two runs (p<0.05). These resultswere confirmed by the sludge reduction in two runs inthe blanks (p<0.05). In other words, variation of oxygenconcentration did not affect the sludge reduction effi-ciency in these experiments.