Optimization of algal removal process at Morton Jaffray water works, Harare, Zimbabwe

Physics and Chemistry of the Earth 36 (2011) 1141–1150

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Optimization of algal removal process at Morton Jaffray water works, Harare, Zimbabwe Zvikomborero Hoko ⇑, Patience K. Makado Department of Civil Engineering, University of Zimbabwe, P.O. Box MP167, Harare, Zimbabwe

a r t i c l e

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Article history: Available online 22 August 2011 Keywords: Algae Copper sulfate Morton Jaffray water works Removal efficiency Toxins Water treatment

a b s t r a c t This study was carried out at Morton Jaffray (MJ) water works to identify the most abundant algae species and their concentrations, and to assess the effects of algae on water treatment processes. The study also sought to determine the optimum values of contact time, coagulant and algaecide doses for the removal of algae. Sampling and analysis of parameters studied was according to APHA standards. Jar tests simulated coagulation, flocculation and sedimentation to determine the optimum coagulant and algaecide dose, and contact time for removal of algae. The most abundant algae were blue–green algae. The concentration of all algae in the raw water from Lake Chivero ranged from 875 to 6000 cells/ml. It was found out that performance of MJ in removing algae was poor and the filtration stage appeared to be the most affected. The optimum contact time for the algaecide was 30 min. The contact time and settling time were the most sensitive parameters. Higher algae removal was achieved at lower pH values. It is recommended that the algaecide should be dosed at or near the raw water intake tower to increase the contact time unlike the present practice where it is added at the plant just before sedimentation. The application of granular activated carbon and post-chlorination currently practiced at the water works need optimization to remove/neutralize the toxins released as a result of the effect of the algaecide. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Due to the continuing eutrophication of surface waters, algaerelated problems in water treatment have gained worldwide attention (Hang-Bae et al., 2001). Furthermore (Schmidt et al., 2008) reported that the occurrence of toxic cyanobacteria in water bodies used as raw water for drinking water production is recognized as being an increasing problem worldwide. According to Knappe et al. (2004) and Westrick et al. (2010), the major algae-related problems in surface waters are unpleasant tastes and odors and filter clogging. The occurrence of algae blooms causes several problems including reducing the aesthetics of the water environment, blocking water filtration systems and most seriously releasing toxins into the water (Howard et al., 1996). Increased disinfection by-products (DBP) concentrations and microbial re-growth in distribution systems, as well as the production of toxins by some algae species are other algae-related problems in the drinking water industry (Hang-Bae et al., 2001; Westrick et al., 2010). Algal cells and associated algogenic material are trihalomethane precursors which has resulted in the restriction of chlorine usage (Henderson et al., 2008b). Microcystins are the most prevalent class of cyanotoxins and the most frequently stud⇑ Corresponding author. Tel.: +263 772 33 88 99. E-mail addresses: [email protected], [email protected] (Z. Hoko). 1474-7065/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2011.07.074

ied (Westrick et al., 2010). Several of the more common species of cyanobacteria produce potent toxins, which have been shown to induce human, and animal health effects which include gastroenteritis, liver damage and neurotoxicity (Dugan and Williams, 2006). Cyanobacteria are often associated with eutrophic or nutrient rich water (Howard et al., 1996). A limited number of algal species also excrete toxic metabolites which, if consumed in sufficient quantities, can cause health problems (Henderson et al., 2008a). The first case of human harm attributed to algal toxins in Zimbabwe as reported in Harare in the mid 1960s later led to the fact that children living in an area of the city supplied from a particular water reservoir developed gastroenteritis each year at the same time a natural bloom of Microcystis was decaying in the reservoir (Falconer, 1999). Passage of cyanobacterial cells through the water treatment process, followed by cell lysis and toxin release, is one potential route of human exposure (Dugan and Williams, 2006). The potential for toxin release by cyanobacteria, in particular from Microcystis, has resulted in the World Health Organization setting a guideline value of 1 lg/l for the associated toxin, Microcystin-LR (Henderson et al., 2008b). Excessive amounts of algae have seriously impacted on the raw water abstraction and water treatment at Morton Jaffray (MJ) water works (Moyo and Mtetwa, 2002), especially the filtration process as evidenced by the increased backwashing frequency now reported to be at every 4–8 h daily. The increased backwashing frequency

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has resulted in reduced plant output (as the filters are the only stage operated in a discontinuous manner due to the need to backwash the filter) and consequently contributing to water shortages in Harare and its satellite towns. The water shortages have resulted in increased cases of water borne diseases. Cholera cases linked to water and sanitation have been reported frequently with the most severe having been in 2008/9 with about 4300 deaths. The high levels of algae also have a cost implication due to the resultant high chemical demand in the different stages of treatment. Maya (1996) reported that the aluminium sulfate dosage for coagulation during potable water treatment increased from 35 to 40 mg/l in 1991 and to 75–80 mg/l in 1992 and then to 100 mg/l in 1995 as a result of the deterioration of the raw water quality (Lake Chivero) for the city of Harare. About seven chemicals in high dosages are now used at MJ compared to an average of three several years back. Most of the chemicals are imported against a background of the country facing serious economic challenges, which has affected the general operation of water and wastewater infrastructure in Zimbabwe. The most effective means of eliminating algae, Microcystins and other toxins would be to control the algal blooms in the lake, through reduction of the nutrient supply to the lake (Chorus and Bartram, 1999). This would require a major capital investment in sewage treatment works and in the disposal of sewage effluent. The investment in water and wastewater infrastructure has been limited with the current economic situation in Zimbabwe. For example the expansion of the water supply infrastructure by the construction of Kunzwi Dam east of Harare, associated water treatment works, transmission pipelines and ancillary works has not been possible due to financial constraints among other things since its proposal over 15 years ago. In the short-term algae removal at the water works has been the solution. The design of conventional water treatment plants, with pH adjustment, pre-chlorination, flocculation by aluminium sulfate and polyelectrolyte, sedimentation, rapid sand filtration, post-chlorination treatment, is effective in removing intact cyanobacterial cells (Falconer, 2005)., but ineffective in removing dissolved toxin (Falconer, 2005; Jurczak et al., 2005). Westrick et al. (2010) suggest that one strategy to minimize algal content in raw water abstracted is to adjust intake depth at specific times to avoid drawing contaminated water and cells into the treatment plant. Morton Jaffray intake tower in Lake Chivero has multiple intake points over the depth of the intake tower. However cyanobacteria can regulate their buoyancy in a diurnal pattern, thus knowledge of bloom ecology and water column dynamics is essential for utility water managers (Westrick et al., 2010). Chemical control may be used as an emergency measure of the control of algae, usually by use of algaecides (Chorus and Bartram, 1999). Compounds that have been used as algaecides include copper sulfate, potassium permanganate, chlorine and ferric sulfate (McKnight et al., 1983; Hart et al., 1997). It has been reported that oxidants have the ability to function as a coagulant aid to enhance micro-flocculation and to reduce conventional coagulant dosages of which common oxidants are chlorine, chlorine dioxide, ozone, iodine and potassium permanganate (Shehata et al., 2002). Pretreatment, using oxidants such as ozone, chlorine, potassium permanganate and potassium ferrate, has been shown in many instances to improve algae removal as a result of ‘‘algal inactivation’’ (Henderson et al., 2008b). The most commonly used algaecide is copper sulfate according to Howard et al. (1996). A traditional treatment to control and prevent phytoplankton development is the use of copper sulfate because of its effectiveness as an algaecide and its low cost (Hadjoudja et al., 2009). In a study by Henderson et al. (2008a), which covered eight plants in the UK, in which pre-oxidation was by ozone followed by coagulation by Alum or ferric chloride t was found out that clar-

ification following coagulation was mostly achieved by dissolved air flotation (DAF) not sedimentation, as would have historically been the case. Coagulation, flocculation, and dissolved air flotation (DAF) is more effective than sedimentation for cyanobacteria-rich waters and this has been the conclusion by several studies as reported by Westrick et al. (2010). The application of powdered or granular activated carbon against dissolved toxins is common (Schmidt et al., 2008). Microcystin can be absorbed by activated carbon with high mesopore capacity (Westrick et al., 2010). A larger proportion of intracellular toxin is released into the water when bloom collapse occurs (Westrick et al., 2010). Bloom collapse occurs when environmental conditions become unfavorable for cyanobacteria, such as the lack of nutrients, flow/mixing changes, or when algaecides are added to water during treatment. Thus the use of chemical oxidants results in release of toxins by the dead algae thus creating a risk of the toxins getting to the consumer. According to Merel et al. (2010), chlorination can be an efficient remedial measure for most of the cyanotoxins however indications from other studies show that further investigation is required to consider chlorinated toxins according to global toxicity tests. However, in most cases, chlorination seems to surprisingly reduce mixture acute toxicity (Merel et al., 2010). For a better management of cyanotoxins in drinking water treatment, chlorination has to be integrated in a multi-barrier approach including, for example, adsorption on activated carbon (Merel et al., 2010). This paper is based on a broad objective to determine the impacts of algae on water treatment processes and the conditions suitable for optimizing control of the algae during treatment. The specific objectives of the study were to; identify the most abundant algae species and their levels, and to assess the effects of algae on water treatment processes. The study also sought to determine the optimum values of contact time, coagulant and algaecide doses for the removal of algae. 2. Study area The Harare Metropolis, which lies upstream of Lake Chivero, in the upper Manyame Catchment, consists of the city of Harare, and its satellite towns of Chitungwiza, Ruwa, Norton and Epworth. The city and its satellite towns lie within the catchment area of the major sources of water supply (Lake Chivero and Lake Manyame) and as a result the drainage from the city and towns flows into the water supply lakes (Nhapi et al., 2002). According to Gumbo (2005) the existing urban water and drainage system is a singleuse-mixing system where water is used and discharged to waste. Wastewater is flushed into sewers and after some treatment, the effluent is discharged to the main drinking water source, Lake Chivero either directly through river discharges or indirectly as runoff from tertiary land treatment systems, which are mainly irrigation of pastures at City of Harare farms. 2.1. Lake Chivero Lake Chivero is the main supply reservoir for Harare and its satellite towns. The Lake was constructed in 1952 for the supply of water to City of Harare as well as irrigation commitments to nearby farms. Lake Chivero catchment has an area of 2136 km2 (JICA, 1996) and has a capacity of 247 Mm3 (Nhapi et al., 2002). It has been the subject of a number of studies in the past years with a common focus on the deterioration water quality (Munro, 1966; Nduku, 1978; Thornton, 1982; Moyo, 1997; Ndebele and Magadza, 2006). In the mid to late 60s the lake became hypereutrophic, with odors of rotting blue green algae, especially anabaenopsis, emanating from the lake (Thornton, 1982). Eichhornia crassipes, the water hyacinth also appeared in the lake and spread at an alarming rate

Z. Hoko, P.K. Makado / Physics and Chemistry of the Earth 36 (2011) 1141–1150

(Moyo, 1997). Other species, such as Asterionella and Ulothrix variabilis, have also been detected in Lake Chivero (Johansson and Olsson, 1998). Studies carried out during that period concluded that sewage effluent was the main source of nutrient enrichment in the lake. Other studies have also shown that the major source of pollution for Lake Chivero is sewage effluents (Nhapi et al., 2002; Nhapi and Hoko, 2004). Though sewage is the most identifiable source of pollution, contributing about 40% of nutrient input into the lake, non-point sources also contribute a significant amount of nutrients (Nhapi et al., 2002; Gumbo, 2005). According to Hranova et al. (2002), about 60% of the treated effluent is reused indirectly for potable application by release to rivers which drain into Lake Chivero. The high nutrient levels in Lake Chivero have also resulted in the proliferation of blue–green algae (Mathuthu et al., 1997). The algae has caused problems, for example filter clogging and high chemical demand, in potable water treatment at Morton Jaffray water works in Harare (Nhapi et al., 2002; Gumbo, 2005). 2.2. Morton Jaffray water works Morton Jaffray (MJ) water works is about 35 km to the Southwest of Harare. Fig. 1 shows the location of MJ water works. MJ water treatment plant has three treatment units, the oldest unit (unit 1) having been built in 1954. Units 2 and 3 were constructed in 1976 and 1994 respectively. The design capacities in cubic meters (m3) per day for units 1–3 are 160,000, 227,000 and 227,000 respectively, making a total of 614,000 m3/day. The treatment units were constructed when pollution was still low and by then water treatment required only chemicals such as aluminium sulfate, lime and chlorine. At present about seven chemicals (activated carbon, sulphuric acid, algae-kill 2500, aluminium sulfate, sodium silicate, chlorine and hydrated lime) are used at high dosages, further confirming that the raw water quality from the source (Lake Chivero) has deteriorated. The water treatment flow scheme is shown in Fig. 2. The raw water from Lakes Chivero and Manyame is generally mixed in the ratio 2:1 respectively. In the mixing chamber, granulated activated carbon (GAC) is added, the doses ranging from 10 to 50 mg/l. Sulphuric acid (H2SO4) is also added in the mixing chamber to lower the pH so as to decrease the chemical demand for coagulation. From the mixing chamber, the water flows into a distribution chamber from where it flows into six baffled channels, each leading to a clarifier. Hydrated aluminium sulfate (coagulant), activated silica (coagulant aid) and algae-kill 2500 (algaecide) are added into each channel. Aluminium sulfate doses range from 65 to 80 mg/l, whilst algae-kill 2500 doses is about 0.8–1.2 mg/l. The water then passes through a clarification process before flowing into the rapid sand filters for filtration. After filtration the treated water passes through a lime and chlorine dosing chamber for disinfection and pH control. The water is then pumped to storage reservoirs before distribution to consumers. Apart from the high chemical demand some of the problems at MJ include frequent backwashing frequency reported to be after every 8 h due to algae and hydraulic loading and at times the filter run time is reduced to 4 h. 3. Materials and methods 3.1. Treatment plant study The study design included monitoring of water treatment processes (treatment efficiency) at key stages of Morton Jaffray Works (Fig. 2) and laboratory based simulations of the treatment process up to the sedimentation stage under different operation conditions. Monitoring was done on units 2 and 3, which have a total de-

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sign capacity of 454 ML per day. Parameters monitored included the types and concentrations of algae, turbidity, and pH. Grab samples were collected from the sampling points at the plant (PK1– PK6). Turbidity and pH were measured according to standard methods suggested by APHA (1998). Samples for algae were prepared and analyzed according to the APHA (1998) standards 10200C and D. 3.2. Laboratory simulations The effective coagulation conditions for algae removal, as well as the impacts of algaecide dose and contact time were investigated by simulations of the stages from the raw water intake at the tower in Lake Chivero, algaecide dose, coagulation and flocculation, and sedimentation using a flocculator (jar test equipment) in the laboratory. Hang-Bae et al. (2001) and Shehata et al. (2002) performed jar test to investigate the efficiency of different coagulants to remove algae. The simulated scenarios are presented in Tables 1–3. For these simulations, raw water from Lake Chivero was used, to represent an untreated sample before any chemicals are added. In practice the raw water intake at MJ inlet works is a blend of the water from Lake Chivero and that from another downstream lake (Manyame) of better quality water. The ratio of the blended water (Lake Chivero: lake Manyame) is on average 2:1 due to restricted water rights for Lake Manyame. There is no access to collect the Lake Chivero-Lake Manyame blend before addition of GAC and sulphuric acid due to the inlet design. However as the water from Chivero is generally of a poorer quality than that of Lake Manyame and consequently of the blend, findings of this study are deemed to be conservative. The current operational practice at MJ, where the algaecide and coagulant are dosed at approximately the same time after the addition of GAC was simulated as scenario 0. Scenario 1 represented a scenario whereby the contact time between the algaecide and the algae could be increased to about 30 min1 by applying the algaecide near the intake tower located in Lake Chivero. A similar simulation to scenario 1 but having the contact time as 45 min was (scenario 2) simulated. Apart from increasing the contact time between the algae and algaecide, scenarios 1 and 2 if implemented will result in the reduction of the interference of other chemicals such as granular activated carbon and alum as these are reported to affect the algaecide efficiency. In all simulation scenarios GAC was not added. The water treatment process at Morton Jaffray could not be manipulated for study owing to the sensitive nature of water treatment and the possible impacts on human health associated with problems in water treatment. As such only laboratory simulations were used to investigated algae removal under different operational conditions. The plant performance was studied under the operational conditions set during the study period. 4. Results and discussion 4.1. Types and levels of algae Results of this section are based on the analysis of raw water samples, samples after key water treatment stages at MJ and samples from the treatment plant and consumer taps. The raw water had different types of algae as shown in Table 4. Anabaena species were observed to be the predominant algae detected in the clarified water (at the water works) even when other species existed in the raw water. Anaebena was found in all samples taken from the water works and was not found only 1 This scenario is based on the assumption of a flow velocity of 1 m/s from the intake tower which is about 1800 m from the inlet works of the plant.

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Fig. 1. Location of study area (Morton Jaffray water treatment works).

in tap water. According to a survey carried out by Knappe et al. (2004) in the USA, 60 out of 114 water treatment plants surveyed experienced filter clogging, 80% of which related filter clogging to the presence of Anabaena, Microcystis and Ankistrodesmus. Anabaena has been known to cause filter clogging in many water treatment plants (Hang-Bae et al., 2001; Knappe et al., 2004). Rapid filter clogging and reduced filter run times reduce water production since clean treated water is used for the backwashing process. Harare is currently facing water supply challenges which have been attributed to among other things, treatment works output and chemical requirements which have been linked to algae in Lake Chivero, the main raw water source. Filter run times have been reported to be as low as 4–8 h at MJ compared to recommended values of 24–36 h. The occurrence of Synedra spp. in the source water of the CheongJu water treatment plant (South Korea) decreased filter run times of rapid sand filters to under 5 h (Hang-Bae et al., 2001). A concentration of 2700 cells/ml of the filamentous diatom, Melosira, at Wahnbach Reservoir, Germany, resulted in reduction of the filter run time from 30 to 8 h, which was further reduced to 4 h as a result of a simultaneous influx of smaller cyanobacteria Coelosphaerium naegelianum (Henderson et al., 2008b).

According to Chorus and Bartram (1999), Microcystis and Anabaena are known to produce unpleasant odors due to their ability to produce odor and taste causing compounds such as geosmin and 2-methylisoborneol (2-MIB). A survey in the US showed that 90% of water treatment plants under survey reported taste and odor problems, and related them to the presence of algae (Knappe et al., 2004). Anabaena has been associated with geosmin and 2MIB episodes in Japan (Ashitani et al., 1988). Microcystis is known to release toxins such as hepatotoxins, hence posing a health risk to the consumers (Chorus and Bartram, 1999). According to Collins (1978), of all the algae in the Cyanophycophyta with toxic properties, Microcystis aerugiosa is probably the one that causes the most harm. Toxins are structurally diverse and their effects range from liver damage, including liver cancer, to neurotoxicity (Hitzfeld et al., 2000). Although the amount of toxins in the water was not investigated in this study, previous studies on Lake Chivero, by Johansson and Olsson (1998) and Ndebele and Magadza (2006) have shown that there were high levels of toxins (13.9 lg/l and 19.86 lg/l respectively), especially in the raw water, associated with Microcystis spp. The World Health Organization has recommended a limit of 1 lg/L of Microcystin in drinking water, based on Microcystin-LR (Hitzfeld et al., 2000; Fleming et al., 2002.).

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The presence of taste and odor algae has a cost implication because of the increased demand for GAC to remove the odors and the presence of different taste and odor algae may cause odor problems at MJ water works. During the period of study, records at one of the water supplier’s laboratories indicated bad taste and odor complaints by consumers. Consumer perceptions on the drinking water quality were however not studied. 4.2. Removal of algae at MJ Results of this section are based on algae counts from samples collected from the water sources and from various stages of Morton Jaffray water treatment works. The algae concentrations at the different water treatment stages during the period of study are summarized in Table 5. The raw water total algae counts ranged from 875 to 6000 cells/ml of water. Algae was found to be present in all stages of water treatment, and even in the distribution system (PK6). This indicates inability of the plant to effectively remove algae. Possible reasons for this include hydraulic overloading of the plant during the study period, inappropriate algaecide dosage and low sand levels in the filters. Thus the drinking water supplied posed health risks to the consumers. Studies elsewhere reported raw water algae concentrations of 400–2700 cells/ml (Hang-Bae et al., 2001). The average cumulative removal for the combination of coagulation, sedimentation and filtration was 94.9% and in 7 days out of the 8 sampling days (87.5% of cases), the cumulative efficiency was greater than 90%. The combination of coagulation, sedimentation and filtration removes more than 90% of algae (Yoo et al., 1995). It appears GAC is removing the bulk of the algae as an average overall efficiency of 60% of the algae was removed after the addition of GAC (up to PK3). GAC especially that with high mesopore capacity has been applied in drinking water treatment for the removal of algae and algal toxins (Schmidt et al., 2008; Westrick et al., 2010). The average cumulative removal efficiency up to sedimentation was 93.8% against a possible 80% according to Chorus and Bartram (1999). The 80% cited from literature are achieved when GAC and

Fig. 2. MJ water treatment flow scheme and sampling points.

Table 1 Laboratory simulation of the current MJ operational practice (scenario 0). Stage

Mixing intensity (rpm)

Mixing time (t) (min)

Chemical additions

Rapid mix Flocculation Sedimentation

200 20 0

3 15 30

Coagulant (Alum) at t = 1, algaecide at t = 3 Flocculant aid (LT 25) added after 5 min of flocculation None

Mixing intensity as measured by the speed of revolution of the mixture and mixing time were as per the standard recommendations of the jar test experiment.

Table 2 Laboratory simulation of application of the algaecide at Lake Chivero intake tower (scenario 1). Stage

Mixing intensity (rpm)

Mixing time (t) (min)

Chemical additions (t in minutes)

Comment

The inertia of the raw water in the pipeline assumed to be equivalent to 20 rpm in the flocculatora. This simulation is for 30 min contact time of the algaecide before other chemicals Mixing intensity and mixing time are as per the standard recommendations of the jar test experiment Mixing intensity and mixing time are as per the standard recommendations of the jar test experiment No mixing and settling time are as per the standard recommendations of the jar test experiment

Flow from source to the WTP Rapid mix

20

30

Algaecide at t = 0,

200

1

Coagulant (Alum)

Flocculation

20

15

0

30

Sedimentation

Flocculant aid (LT 25) added after 5 min of flocculation None

a The inertia of the water in the pipeline between the intake tower in Lake Chivero and the inlet works at the plant has been assumed to be equivalent to that generated at 20 rpm in the flocculator. This inertia is just an estimate and may need refinement and further investigation by determining the equivalent G value in the flocculator to inertia in the abstraction pipeline.

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Table 3 Laboratory simulation of application of the algaecide at Lake Chivero intake tower (scenario 2). Stage Flow from source to the WTP Rapid mix Flocculation Sedimentation

Mixing intensity (rpm)

Mixing time (t) (min)

Chemical additions (t in minutes)

Comment

20

45

Algaecide at t = 0,

200

1

Coagulant (Alum)

20

15

0

30

Almost similar to scenario 1 but 45 min contact time of the algaecide before other chemicals Mixing intensity and mixing time are as per the standard recommendations of the jar test experiment Mixing intensity and mixing time are as per the standard recommendations of the jar test experiment No mixing and settling time are as per the standard recommendations of the jar test experiment

Flocculant aid (LT 25) added after 5 min of flocculation None

Table 4 Types of algae from samples taken at MJ and consumer tapes in the period 8 March–22 May 2007. Location

Algae types

Raw water (PK1 and PK2) Distribution Chamber (PK3) Clarified water (PK4) Filtered water (PK5) Final treated tap water (PK6)

Anabaena, Cloesterium, Cyclotella, Microcystis, Pandorina, Volvox, Ankistrodesmus, Chlamydomonas, Eudorina, Scenedesmus Anabaena, Cloesterium, Microcystis, Volvox, Ankistrodesmus, Chlamydomonas, Eudorina Anabaena, Microcystis, Ankistrodesmus Anabaena, Microcystis, Ankistrodesmus Microcystis

algaecide are not applied. The average stage removal across the sedimentation stage was 76.1%. Henderson et al. (2008b) in a review article deduced that settlement achieved between 70% and 80% removal efficiency for settlement times varying from 10 min to 2 h when using aluminium sulfate for a variety of species. In our study the range of stage efficiency for the rapid sand filters was 5.7% to 5.9% with an average of 1.7%. The negative removal efficiencies suggest breakthrough of the algae. It is possible that there is rapid reproduction of the algae in the supernatant water and this could possibly account for the increase of algal cells in the filter effluent. Lepistö et al. (1992) found the algal removal efficiency of rapid sand filters as 14% in an evaluation of full scale treatment plants. Reported full-scale filtration efficiencies of 99% and 85% for Anabaena circinalis and Microcystis aeruginosa, respectively (Dugan and Williams, 2006) Rapid filtration after coagulation does not effectively remove cyanobacterial cells (Jurczak et al., 2005). It is therefore concluded that the filter efficiency at MJ was very low. Some of the reasons include low sand level which was below 50 cm (against a recommended 1 m) for some of the filters and overloading of the plant. Dugan and Williams (2006) demonstrated that filter breakthrough of algae was higher at higher filter loading rates compared to lower ones. The total algae concentration was 875–6000 cells/ml in raw water, 75–225 in filtered water and 1–125 in tap water. These levels were high compared to recommended levels suggested in literature. Therefore the performance of MJ in removing algae was poor and the filtration stage appeared to be the most affected.

4.3. Determination of optimum contact time, coagulant and algae dosage 4.3.1. Effect of coagulant dosage on algae removal The results of this sub-section are based on the laboratory simulations of the coagulation, flocculation and sedimentation stages using a flocculator (jar test equipment) in a laboratory. A standard flocculator with six beakers of 1 L volume was used. Jar tests were used to determine the conditions that lead to improved algae removal. To demonstrate the dependence of algae removal on coagulant dose, jar tests in which only alum, at varying doses, was added to each beaker, were carried out as per the guidelines. Algae concentration and turbidity were then determined for the settled water. Fig. 3 presents results of the settled water algae counts

and turbidity as a function of alum dose, at a starting pH of 7.5 (raw water pH). Algae counts and turbidity measured at the end of the experiment reduced with increasing alum dosage. Though low alum dosages of around 60 mg/l could remove turbidity, to achieve the maximum allowable turbidity by WHO guidelines (WHO,1996) and Zimbabwe standards (5NTU and 1NTU respectively), a significant amount of algal cells (i.e. about 1600 cells/ ml) still remained in the clarified water at the end of the tests. Studies conducted by Knappe et al. (2004) showed that algae counts decreased with increase in alum dose. Fig. 3 also shows that algal cell removal increased with increase in alum dose. No significant turbidity occurred after a dosage of 65 mg/l and no further significant algae removal was achieved after a dosage of 90 mg/l. This possibly suggests that removal of algae is achieved at higher levels of coagulant dosage compared to turbidity. At the dose of 90 mg/l the algal removal efficiency was about 88%. In lab experiments using the jar test equipment the efficiency of two chemical coagulants (alum and ferric chloride) used for removing Nile water algae was 79% for alum and 89% for ferric chloride (Shehata et al., 2002). The combination of coagulation, flocculation and sedimentation, the processes simulated by the jar tests has been reported to achieve an overall efficiency greater than 80% (Chorus and Bartram, 1999). Also it suggests that some of the algal species may not be contributing to turbidity. Elevated concentrations of alum in potable water has been indicated as a causative agent of neurological diseases, such as Alzheimer’s disease and presenile dementia (Shehata et al., 2002). It appears that for the removal of algae, the alum dosage should be 65–90 mg/l. At the time of study the average coagulant dosage was 65–80 mg/l. The effect of pH at which coagulation is carried out on algae removal was examined by performing jar tests at initial pH of 7.0 and 7.5. Fig. 4 shows the percentage removal of algae at pH 7 and 7.5 and different coagulant doses. Algae removal was more effective at pH 7 compared to pH 7.5. At both pH values of 7.0 and 7.5 the removal efficiency of algae increased with increasing dosage of alum. There was no further significant increase2 in removal efficiency beyond a dosage of 90 mg/l when the starting pH was 7 and at dosages beyond 110 mg/l at a starting pH of 7.5. Alum works in the range 6.0–8.0 and coagulation

2 Here no significant increase is when the difference between removal efficiencies of successive dosages is less than 1%.

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Z. Hoko, P.K. Makado / Physics and Chemistry of the Earth 36 (2011) 1141–1150 Table 5 Average algae concentration at MJ for the period 26 February–16 April 2007. Stage

Concentration of algae (total count in cells/ ml) n = 6

Range of stage removal efficiencies

Range of overall stage removal

Range of cumulative removal at stage

Raw water (PK1 and PK2) Distribution chamber water (PK3) Clarified water (PK4) Filtered water (PK5) Final treated (PK6)

875–6000 (2848 ± 1605) 225–3150 (1150 ± 1116)

0 18.9–85.8 (60.8 ± 27.8)

0 18.9–85.8 (60.8 ± 27.8)

0 18.9–85.8 (60.8 ± 27.8)

65–180 (128 ± 37) 75–225 (117 ± 49) 1–125 (54 ± 44)

33.3–94.3 (76.1 ± 23.2) 92.3 to 50.0(0.8 ± 50.0) 66.7 to 99.2 (46.8 ± 53.8)

7.4–76.5 (33.1 ± 29.0) 3.7 to 8.6 (1.0 ± 3.7) 5.7 to 5.9 (1.7 ± 3.5)

82.9–98.3 (93.8 ± 4.8) 88.9–98.0 (94.9 ± 3.1) 85.7–100.0 96.5 ± 4.8)

Figures in brackets are the average ± standard deviation. Stage removal is the removal efficiency at the treatment stage i.e. comparing the influent and the effluent for that stage. Overall stage removal is the removal efficiency at the stage when comparing the difference between the influent and effluent across the stage to the raw water quality.

is more effective at lower pH due to the strong ionic concentration of the solution which promotes amalgamation (Degrémont, 1991). Tests have shown that algae removal is improved by lowering pH (AWWA, 1990). Proper coagulation conditions can greatly reduce the algae load to rapid sand filters. Again, because intact algal cells are removed by coagulation, followed by sedimentation, removal of intracellular toxins and odor causing compounds can be achieved Hang-Bae et al. reported high removal of algae when alum was used at pH 6.8–7. It was found out that algae and turbidity removal increased with increasing coagulant dosage. More coagulant dosage was required to remove algae compared to turbidity. Algae removal by coagulation was found to be higher at lower pH and in this study it was higher at pH 7 compared to pH 7.5.

4.3.2. Effect of algaecide dose and contact time on algae removal Jar tests were carried out to determine the effect of contact time and algaecide dose on the removal of algal cells. The actual mechanisms of the effect of copper on algae was not studied in this research. However, studies have shown that Cu ions prevent cell division and causes accumulation of the products of photosynthesis and thus the depression of photosynthesis (Mcknight et al.,1983). According to an assessment of the use of CuSO4 for the control of cyanobacteria by Mcknight et al. (1983), it was found that there is a wide difference in Cu sensitivity among algae species. The relative growth inhibiting concentrations for a range of algae are given in terms of cupric ion activity (i.e. [Cu2+]). According to Hadjoudja et al. (2009), copper toxicity is ascribed to the free metal ion concentration for metal–organism interactions according to the free ion activity model. The relative toxicity is given in terms of ionic copper because algae react principally to the concentration of Cu2+ or loosely complexed copper rather than the total dissolved metal in the water. Most filter clogging and taste and odor algae are susceptible to CuSO4. Table 6 and Fig. 5 show results of the effect of algaecide dose and its contact time on the removal of algae, at different alum doses. The results show that the contact time between algaecide and algae is important for effective removal of algae. The removal efficiency increased with contact time allowed for the algaecide before coagulation for each combination of algaecide and alum. For example for the scenario 0 (negligible contact time between algaecide dosage and alum dosage) the removal efficiency ranged from 59.68% to 93% while for scenario 1 the efficiency ranged from 91.39% to 98.8% and 93.3% to 98.9% for scenario 2, for algaecide dosage ranging from 0.05 to 1 mg/l and coagulant dosages ranging from 65 to 110 mg/l. Shehata et al. (2002) found that the efficiency of oxidants for Nile water algal removal increased when the iodine, ozone or potassium permanganate was combined with alum, and the average algal reductions were 91%, 91.4% and 95.2%, respectively. In a lab study of algal removal in which coagulants were

mainly used, a higher degree of algal removal took place when pre-chlorination was added and reached 91% and 95% for alum and ferric chloride, respectively (Shehata et al., 2002). Thus the algal removal efficiency by a combination alum and copper sulfate found in this study compare well with results of experiments with alum combined with other oxidants. From Fig. 5 it can be seen that algaecide dosage is not a sensitive parameter but the contact time. Also at higher contact times the change in removal efficiencies over the whole range of the coagulant dosage studied was lower compared to that at lower contact times implying that the coagulant dosage is not a sensitive parameter at higher contact times of the algaecide and the algae. At 30 min and 45 min contact times the removal efficiency was greater than 90% for all values of coagulant doses including the lowest value of 65 mg/l. The algaecide dosage appeared more sensitive at low retention times (scenario 0) as significant differences were noticed between successive algaecide dosages only under this scenario. The alum dosage was also a sensitive parameter under scenario 0. For retention times of 30 min and 45 min there was no noticeable change in removal efficiency between successive algaecide dosages implying that under the study conditions and scenarios, 30 min contact time could be optimum. This suggest that at retention times of at least 30 min (which is recommended by Chorus and Bartram (1999)) dosages of as low as 0.05 mg/l appear to be achieving the same result as dosages 20 times higher (i.e. 1 mg/l). Studies with other chemicals and oxidants used to remove algae and algae related problems have also shown that the contact time with the target organism is important. Again for each algaecide dosage the removal efficiency of algae increased with increasing coagulant dosage and likewise for each coagulant dosage the removal efficiency of algae increased with increasing algaecide dosage. Results of a study by Hadjoudja et al. (2009) showed that regardless of the cell type, as copper concentrations increased, cell division rate decreased. Gibson (1972) concluded that the lethal dose of copper as copper sulfate is between 0.8 and 1 mg/l, but for a rapid algaecidal effect, a larger dose would be required. The treatment levels by copper are generally in the range 0.025–1 mg (Cu) l 1(Hadjoudja et al., 2009). Overdosing cannot only induce cell lysis, releasing undesirable toxins or taste and odor compounds, but also degrade extracellular organic material (EOM) to the extent that compounds with interfering properties including mono and dicarboxylic acids and glycaric acids are formed (Henderson et al., 2008b). Westrick et al. (2010) cited a number of studies which have concluded that KMnO4 can be used at the intake without releasing intracellular saxitoxins or taste and odor compounds. It was concluded that removal efficiency increased with increasing contact time and also with increasing algaecide dosage. The contact time was found to be a more sensitive parameter

Z. Hoko, P.K. Makado / Physics and Chemistry of the Earth 36 (2011) 1141–1150

Algae (cells/ml)

3500 3000 2500 2000 1500 1000 500 0

Turbidity (NTU)

6 5 4 3 2 1 0

0

25

50

75

100

Turbidity (NTU)

Algae concentration (cells/ml)

1148

125

Alum dose (mg/l)

% removal of algae

Fig. 3. Effect of alum dosage on settled water turbidity and removal of algae at pH 7.5.

100 80 60 40 pH 7.5

20

pH 7

0 0

20

40

60

80

100

120

alum dose (mg/l) Fig. 4. Effect of coagulant dose and coagulation pH on removal of algae.

compared to algaecide dosage. The optimum contact time under the study scenarios was found to be 30 min (scenario) in lab experiments while the optimum algaecide dosage was 0.05 mg/l.

4.4. Effect of settling time on algae removal Results of this section are based on jar test simulations. Another important issue in the removal of algae in drinking water treatment is the settling time after coagulation. Fig. 6 shows the settled water algae counts after different settling times obtained with initial algae concentration of 4500 cells/ml before application of any chemical. Samples were collected at predetermined time intervals at starting with the moment the flocculator mixers were switched off (t = 0) until 30 min after the mixtures were switched off. After coagulation and flocculation, at 0 min settling time, an algae count of 2500 cells/ml (44% reduction) was obtained. The algae count at this stage shows that some of the algae could have been incorporated in the flocs during coagulation and flocculation thus

scenario 0 (0.05) scenario 0 (1 mg/l) scenario 1 (0.8) scenario 2 (1 mg/l)

scenario 0(0.1) scenario 1(0.05) scenario 1 (1mg/l)

scenario 0 (0.8) scenario 1 (0.1) Scenario 2(0.8)

100

concentrated at lower levels. Algae removal was 68% for a settling time of 10 min and 73% after 15 min of settling. Algae removal was observed to increase with settling time. A removal efficiency of 91% was achieved after 30 min of settling, the recommended settling time in the flocculation test. As earlier suggested by Yoo et al. (1995), the combination of coagulation, sedimentation and filtration removes more than 90% of algae. The settling time is therefore important in effective removal of coagulated algae. Where treatment units are overloaded, settling time is reduced. This was the case with MJ water works which was receiving 550,000 m3/day for units 2 and 3 against a rated capacity of 454,000 m3/day. This reduced settling time as the retention time would be lower in the settling tanks. Knappe et al. (2004) investigated the settleability of algae-laden floc for falls lake water in the USA and they reported that algae removal was poor for settling times of 10 min. At 20 min the observed algae removal was in excess of 50%. Findings of this study seem to agree with those by Knappe et al. (2004). It was concluded that algae removal increased with increasing settling time. Settling times of 30 min in the lab experiment which are the equivalent of the actual settling times in full scale treatment plants, yielded removal efficiencies of 90% which is what is suggested in literature as the possible removal efficiency up to the sedimentation stage. The optimum settling time was therefore 30 min under laboratory set-up.

no of algae cells [cells/mm]

90 85 80 75 70 65 60 55 65

75

85

95

105

Alum dosage (mg/l) Fig. 5. Effect of retention time on algae removal.

115

3000

100

2500

80

2000 C = 2670.7e

1500

60

-0.0588t

2

40

R = 0.98

1000

20

500 0

0 0

5

10

15

20

25

30

35

40

settling time Total algae cells (cells/ml)

% settled algae

Fig. 6. Effect of settling time on settled water algae counts.

% settled algae

Algae removal [%]

95

1149

Z. Hoko, P.K. Makado / Physics and Chemistry of the Earth 36 (2011) 1141–1150 Table 6 Simulated results for the effect of algaecide dose and contact time on removal of algae. Simulation

Algaecide dose (mg/l)

Raw water algae count

Settled algae count: cells/ml (in brackets) and % removal of algae Alum dose (mg/l) 65

Alum and algaecide dosed at the same time (scenario 0)

0.05 0.1 0.8 1

(4960) (4960) (4560) (4560)

(2000) (1760) (1540) (1500)

30 min contact time before coagulation (scenario 1)

0.05 0.1 0.8 1

(4760) (4760) (5000) (5000)

(410) (360) (355) (320)

45 min contact before coagulation (scenario 2)

0.8 1

(4500) (4500)

(300) 93.33% (280) 93.78%

5. Conclusions and recommendations 5.1. Conclusions Based on results of this study the following conclusions were made: (1) The most abundant algae species were blue green algae, particularly Anabaena and Microcystis. Total algae concentrations ranged from 875 to 6000 cells/ml in raw water and 1 to 225 cells/ml in treated water. The presence of algae was linked to the reported frequent filter clogging and possible taste odor problems. (2) The optimum contact time between the algae and algaecide was found to be 30 min under the study conditions. It was found that at appropriate retention times of at least 30 min the algaecide dosage is not a sensitive parameter as the removal efficiencies under the ranges of algaecide dosages applied (0.05–1 mg/l) showed no significant change, suggesting that an algaecide dosage of as low as 0.05 at a minimum contact time of 30 min and alum dosage of 65 mg/l may be sufficient. (3) Settling time was also found to have an impact on the algal removal efficiency. (4) It was also concluded that pH is an important parameter for algal removal and that better results were found at lower pH values i.e. at pH 7 compared to at 7.5. (5) Algal removal increased with increasing contact times increasing algaecide dosage and increasing settling. 5.2. Recommendations (1) It is recommended that the copper sulfate algaecide currently being used, be dosed near the raw water intake tower at Lake Chivero. However the effect of possible toxin release should be investigated and possible measures to reduce them such as GAC and chlorination after algaecide dosage should be optimized s these have been reported in literature as capable of reducing the toxins. (2) There is need for further studies to investigate the full scale findings of the lab simulations on the actual plant.

Acknowledgements This paper presents part of the research results of an MSc study by Patience K. Makado at the University of Zimbabwe under a WaterNet fellowship. The Departments of Civil Engineering and

59.68% 64.52% 66.23% 67.11%

91.39% 92.44% 92.90% 93.60%

80

110

(1200) 75.81% (1100) 77.82% (1000) 78.07% (400) 78.29%

(1154) 76.73% (720) 85.4% (400) 91.23% (45) 93%

(170) (150) (215) (142)

96.43% 96.85% 95.70% 97.16%

(58) 98.78% (90) 98.11% (60) 98.50% 98.80

(200) 95.56% (100) 97.78%

(50) 98.89%

Biological Sciences at the University of Zimbabwe are acknowledged for the logistical and material support. The authors are grateful to ZINWA, which was then managing the water supply system of Harare, for the permission to carry out the study. The authors would also want to thank Mr. W. Gumindoga for assisting in the drawing of the map of the location of the study area. References APHA (American Public Health Association), 1998. Standard Methods for Examination of Water and Wastewater, 20th ed. American Public Health Association, Water Environment Federation, Washington, DC. Ashitani, K., Hishida, Y., Fujiwara, K., 1988. Behavior of musty odorous compounds during the process of drinking water treatment. Water Sci. Technol. 20 (8/9), 261–267. AWWA (American Water Works Association), 1990. Water Quality and Treatment. A Handbook of Community Water Supplies, fourth ed. McGraw-Hill, Inc., New York. Chorus, I., Bartram, J. (Eds.), 1999. Toxic Cyanobacteria in Water: A Guide to their Public Health Consequences, Monitoring and Management. Routledge, London. Collins, C., 1978. Algal toxins. Microbiol. Rev. 42 (4), 726–746. Degrémont, 1991. Water Treatment Handbook, 6th ed. Springer, Paris, France, vol. 1. Dugan, N.R., Williams, D.J., 2006. Cyanobacteria passage through drinking water filters during perturbation episodes as a function of cell morphology, coagulant and initial filter loading rate. Harmful Algae 5, 26–35. Falconer, I.R., 1999. An overview of problems caused by toxic blue–green algae (cyanobacteria) in drinking and recreational water. Environ. Toxicol. 14, 5–12. Falconer, I.R., 2005. Is there a human health hazard from microcystins in the drinking water supply? Acta hydrochim. Hydrobiol. 33 (1), 64–71. Fleming, L.E., Rivero, C., Burns, J., Williams, C., Bean, J.A., Shea, K.A., Stinn, J., 2002. Blue green algal (cyanobacterial) toxins, surface drinking water, and liver cancer in Florida. Harmful Algae 1, 157–168. Gibson, C.E., 1972. The algaecidal effect of copper on a green and a blue–green alga and some ecological implications. J. Appl. Ecol. 9 (2), 513–518. Gumbo, B., 2005. Short-Cutting the Phosphorus Cycle in Urban Ecosystems. PhD Dissertation. UNESCO-IHE Delft University of Technology, Taylor and Francis Group. Hadjoudja, S., Vignoles, C., Deluchat, V., Lenain, J., Jeune, A.L., Baudua, M., 2009. Short term copper toxicity on microcystis aeruginosa and Chlorella vulgaris using flow cytometry. Aquatic Toxicol. 94, 255–264. Hang-Bae, J., Young-Ju, L., Byung-Du, L., Knappe, D.R.U., 2001. Effectiveness of coagulants and coagulant aids for the removal of filter-clogging Synedra. J. Water Supply: Res. Technol.—AQUA 50 (3), 135–148. Hart, J., Fawell, J.K., Croll, B., 1997. The Fate of Both Intra and Extra Cellular Toxins during Drinking Water Treatment. IWSA World Congress, Blackwell Science, Oxford. Henderson, R., Chips, M., Cornwell, N., Hitchins, P., Holden, B., Hurley, S., Parsons, S.A., Wetherill, A., Jefferson, B., 2008a. Experiences of algae in UK waters: a treatment perspective. Water Environ. J. 22, 184–192. Henderson, R., Parsons, S.A., Jefferson, B., 2008b. The impact of algal properties and pre-oxidation on solid–liquid separation of algae. Water Res. 42, 1827–1845. Hitzfeld, B.C., Hoger, S.J., Dietrich, D.R., 2000. Cyanobacterial toxins: removal during drinking water treatment, and human risk assessment. Environ. Health Perspect. 108, 113–122. Howard, A., McDonald, A.T., Kneale, P.E., Whitehead, P.G., 1996. Cyanobacterial (blue–green algal) blooms in the UK: a review of the current situation and potential management options. 1996. Progr. Phys. Geogr. 20 (1), 53–61. Hranova, R., Gumbo, B., Klein, J., van der Zaag, P., 2002. Aspects of the water resources management practice with emphasis on nutrients control in the Chivero Basin, Zimbabwe. Phys. Chem. Earth 27, 875–885.

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