Assessing the feasibility of wastewater recyclingand treatment efficiency of wastewater treatment units

Jie-Chung Lou & Yung-Chang Lin

Received: 15 January 2007 /Accepted: 6 April 2007 / Published online: 15 May 2007# Springer Science + Business Media B.V. 2007

Abstract Wastewater reuse can significantly reduceenvironmental pollution and save the water sources.The study selected Cheng-Ching Lake water treat-ment plant in southern Taiwan to discuss thefeasibility of wastewater recycling and treatmentefficiency of wastewater treatment units. The treat-ment units of this plant include wastewater basin,sedimentation basin, sludge thickener and sludgedewatering facility. In this study, the treatmentefficiency of SS and turbidity were 48.35–99.68%and 24.15–99.36%, respectively, showing the signif-icant removal efficiency of the wastewater process.However, the removal efficiencies of NH3–N, totalorganic carbon (TOC) and chemical oxygen demand(COD) are limited by wastewater treatment processes.Because NH3–N, TOC and COD of the mixingsupernatant and raw water are regulated raw waterquality standards, supernatant reuse is feasible andworkable during wastewater processes at this plant.Overall, analytical results indicated that supernatantreuse is feasible.

Keywords Wastewater recycling . Backwash .

Supernatant . Concentrated liquid .

Water treatment plant

Introduction

The average annual amount of distributed water perTaiwanese resident is only one-sixth of the globalaverage. Therefore, water recycling and reuse becomean urgent issue in Taiwan. Wastewater accounts for2–10% of the total water production in watertreatment plants (Cornwell and MacPhee 2001;Dotremont et al. 1999; Le Gouellec et al. 2004;Nasser et al. 2002; Vigneswaran et al. 1996).Wastewater contains the backwash water from therapid sand filter and the drained muddy water fromthe flocculation/sedimentation basins and the clarifier.Reuse of such wastewater reduces water consumptionfor complying with wastewater effluent standards ofwater treatment plants. Supernatant reuse can signif-icantly reduce effluent volume and achieve the goal oftotal wastewater reduction.

Wastewater reuse facilities are designed andutilized in water treatment plants in most developedcountries. Large quantities of impurities in reusewastewater recycled to raw water may contaminateraw water quality and caused difficulty for subsequentwater purification processes. Generally, wastewater isrecycled and reused via direct and process reuse.

Direct reuse

Direct reuse is themost commonmethod of recycling andreusing wastewater in Taiwan, mainly containing reuse ofbackwash water in rapid sand filters (Arora et al. 2001)

Environ Monit Assess (2008) 137:471–479DOI 10.1007/s10661-007-9782-3

J.-C. Lou (*) :Y.-C. LinInstitute of Environmental EngineeringNational Sun Yat-Sen University,Kaohsiung 804, Taiwane-mail: loujc@mail.nsysu.edu.tw

and supernatant reuse during wastewater treatmentprocess. The former is to build a wastewater basin thatcollects backwash wastewater from a rapid sand filter,and then mixes with raw water for recycling. The latteris to build a sludge thickener to concentrate the drainedmuddy water from the flocculation/sedimentation basinand backwash water from the rapid sand filter. Thesupernatant of sludge thickener is mixed with raw waterfor recycling, and the sludge in the bottom of sludgethickener is then delivered to the sludge treatmentsystem. The cost of direct wastewater reuse is low, butprimarily depends on water quality control systems.Once the supernatant quality fails to achieve recyclingstandards, the recycle volume can be reduced orabandoned (Edzwald and Tobiason 2002).

Process reuse

Process reuse treats the wastewater produced bypurification processes to meet raw water qualitystandards for reuse. This method relies on wastewaterquality; when cost will be higher than that for rawwater and raw water will be adequate, process reusewill be ignored.

Kaohsiung city is the largest city in southern Taiwan,with an area of 1,536,029 km2 and a population of 1.5million. The government-owned Taiwan Water SupplyCorp (TWSC) provides drinking water to approximate-ly 98.94% of the population of Kaohsiung city. Cheng-Ching Lake water treatment plant (CCLWTP) is themajor suppliers of drinking water to Kaohsiung city.

The advanced water treatment units of CCLWTPinclude pre-ozonation, pellet softening, post-ozonationand biological activated carbon (BAC) adsorption. Thepre-ozonation not only functions as a disinfectant, butalso oxidizes organic compounds, destroys algaemetabolism and alters the surface characteristics of

the suspended solids to increase the follow-up removalefficiency of the filtration process (Jekel 1998; Narkisand Schnider 1980). The organic compounds are alsotypically considered to be a precursor of disinfectionby-products (i.e., THMs and HAA5). Pellet softening,also known as fluidizing bed crystallization, FBC, isused as a substitute for traditional rapid mixing,coagulation, sedimentation and sludge dewatering toreduce the hardness of drinking water. The function ofpost-ozonation oxidizes residual organic and disinfec-tion (Siddiqui et al. 1997). The BAC bed wasdesigned following post-ozonation to adsorb smallerorganic compounds, and disinfection by-products(Bouwer and Crowe 1988; Miltner et al. 1992;Rittmann and Huck 1989; Servais et al. 1992).Figure 1 shows the advanced purification units atthe CCLWTP.

CCLWTP provides tap water to more than half ofKaohsiung’s population (Lou et al. 2007). Thewastewater treatment units of CCLWTP includewastewater basin, wastewater sedimentation basin,sludge thickener and sludge dewatering facility.Backwash water produced from the rapid sand filteris recycled to the wastewater basin. The supernatantof wastewater basin flows back to raw water forrecycling. The influx of concentrated liquid from thebottom of wastewater basin and the drained muddywater from the flocculation/sedimentation basin thenflows into the wastewater sedimentation basin fortreatment. The supernatant of wastewater sedimenta-tion basin flows back into raw water for recycling.The bottom concentrated liquid of wastewater sedi-mentation basin then flows into the sludge thickenerfor dewatering. Figure 2 shows the wastewatertreatment units at the CCLWTP. Yet, the water qualityof supernatant mixed with raw water has not beenmonitored. Therefore, this investigation of the water

Fig. 1 The advanced puri-fication units at theCCLWTP

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quality during wastewater treatment processesattempts to identify its efficiency and feasibility ofwastewater recycling.

Materials and methods

Sampling location and frequency

Six sites which include the wastewater basin, waste-water sedimentation basin and sludge thickener at theCCLWTP (Table 1) were sampled. The four samplingdates were Jan. 9, Feb. 13, May 15, and June 27, of2006.

Assessment methods

Water quality data was analyzed to evaluate waste-water treatment efficiency and assess the feasibility ofsupernatant reuse during each treatment process.Analytical results for supernatant water quality wereused to evaluate whether recycled water qualitycomplies with effluent standards. Table 2 showseffluent standards for water treatment plants inTaiwan. Effluent standards for chemical oxygendemand (COD) and suspended solids (SS) for watertreatment plants in Taiwan are 100 and 50 mg/L,respectively. Table 3 shows the quantities and qualityat CCLWTP. Recycled water quantities at foursampling times were 12,200–22,410 CMD, occupiedabout 3.5–5.7% of treated water. From the formula ofto calculate the concentration of recycled water andraw water combined. The formula is as follow:

efficiency %ð Þ ¼ concentration of inflowwastewater � concentration of reused supernatant

concentration of inflowwastewater� 100%

Table 1 Sampling sites at the CCLWTP

Wastewater treatmentunits

Types of wastewater Sampleno.

Wastewater basin Backwash wastewater A1Supernatant reuse A2

Wastewatersedimentation basin

The drained muddy water of flocculation/sedimentation basin mixed with bottom concentratedliquid of wastewater basin

A3

Supernatant reuse A4Sludge thickener Bottom concentrated liquid of wastewater sedimentation basin A5

Supernatant reuse A6

Fig. 2 The wastewatertreatment units at theCCLWTP

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Meanwhile, this study determines whether recycledwater quality complies with rawwater quality standards.Table 4 shows the raw water quality standards forNH3–N, COD and total organic carbon (TOC) inTaiwan are 1, 25 and 4 mg/L, respectively. Thus, fivewater quality parameters, SS, NH3–N, TOC, turbidityand COD, were selected as comparison indicators.Table 5 shows standard methods used for five waterquality analyses.

Results and discussions

Analytical results

Figure 3 shows the SS concentrations during eachtreatment process for the four sampling days. Analyt-ical results indicated that the SS concentration 0.4–407.2 mg/L. Wastewater samples A3 and A5, whichcame from the bottom of concentrated liquid waste-

water, had a high SS concentration and largevariation. Sample A3 and A5 in the latest three andtwo samplings exceeded effluent standards for SSconcentration. Based on effluent standards for watertreatment plants in Taiwan, the SS concentrationshould be <50 mg/L. These two sources cannot bedischarged directly in Taiwan. Thus, whether somewastewater may be reused as raw water is investigated.

Figure 4 shows the NH3–N concentrations duringeach treatment process for the four sampling days.Results indicate that the NH3–N concentration was0.07–2.8 mg/L. From the four samples, the super-natants concentrations A4 (1.2 mg/L) and A6(1.5 mg/L) collected on Feb. 13, and A6 (1.26 mg/L) collected May 15 also exceed the maximum limitof 1 mg/L for raw water quality standards. Sincewastewater treatment is used as a substitute forsedimentation and sludge dewatering in wastewaterrecycling, wastewater treatment units were not effec-tive in reducing NH3–N. Hence, whether supernatantsfrom these two sources mixed with raw water willmeet raw water quality standard is discussed later.

Figure 5 shows the TOC concentrations duringeach treatment process for the four sampling days.Analytical results indicate that the TOC concentration

Table 3 The water quantities and quality at the CCLWTP

Items Sampling dates

Jan. 9 Feb. 13 May 15 June 27

Raw water quantity(CMD)

335,560 368,310 368,600 359,940

Recycled waterquantity (CMD)

12,200 15,500 22,410 19,480

Recycled waterpercentage(%)

3.5 4 5.7 5.1

Turbidity of raw water(NTU)

13.7 9.4 13.1 21.7

Average TOCconcentration of rawwater

1.13(mg/L)

Average Ammonia–Nconcentration of rawwater

0.11(mg/L as N)

Average CODconcentration of rawwater

7.75(mg/L)

Table 4 The raw water quality standards (Taiwan, 2006)

Water quality parameters Maximum limit Unit

Ammonia–N (NH3–N) 1 mg/LChemical Oxygen Demand (COD) 25 mg/LTotal Organic Carbon (TOC) 4 mg/L

Table 5 Standard methods used for five water quality analyses

Water qualityparameters

Analyzing method

suspended solid (SS) 103–105°C drying method(aNIEA W210.57A)

Ammonia–N (NH3–N) Ultraviolet spectrophotometer(NIEA W448.51B)

Total organic carbon(TOC)

flaming/infrared spectrophotometer(NIEA W530.51C)

Turbidity Turbidimeter method(NIEA W219.52C)

Chemical oxygendemand (COD)

Dichromate reflux method (NIEAW515.53A)

a Standard Methods for Water Quality Analysis, TaiwanEnvironmental Protection Agency.

Table 2 Effluent standards for water treatment plants (Taiwan,2006)

Water parameters Effluent limits Unit

Chemical Oxygen Demand (COD) 100 mg/LSuspended solids (SS) 50 mg/L

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Fig. 3 The SS concentra-tions during each treatmentprocess for the four sam-pling days

Fig. 4 The NH3–N concen-trations during each treat-ment process for the foursampling days

Fig. 5 The TOC concentra-tions during each treatmentprocess for the four sam-pling days

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Fig. 6 The turbidity valuesduring each treatment pro-cess for the four samplingdays

Fig. 7 The COD concen-trations during each treat-ment process for the foursampling days

Fig. 8 The variation intreatment efficiency for SSby each treatment processfor the four sampling days

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was 1.81–21.94 mg/L. The supernatants concentrationswith >4 mg/L TOC are as follows: A6 (4.19 mg/L),collected on Jan. 9; A4 (7.52mg/L) and A6 (8.61mg/L),collected on Feb. 13; and, A4 (9.12 mg/L) and A6(11.41 mg/L), collected on June 27, 2006. The highconcentration of TOC was found in backwash waste-water and the bottom concentrated liquid of sedimenta-tion as shown in Fig. 4. The ozonation treatmentprocess added to wastewater treatment lower theconcentration of TOC. Thus, supernatants mixed withraw water are below raw water quality standard isdiscussed later.

Figure 6 shows the turbidity values during eachtreatment process for the four sampling days. Analyt-ical results indicate that the turbidity value was 1.0–412 NTU. Wastewater samples A3 and A5, whichcame from the bottom of concentrated liquid waste-water, had a high turbidity values and large variation.

Figure 7 shows the COD concentrations duringeach treatment process for the four sampling days.

Analytical results indicate that the COD concentrationwas 3.0–118.9 mg/L. Sample No. A1, A3 and A5exceed effluent standards of 1 mg/L. These findingsindicate these wastewater constituents cannot bedischarged directly. Therefore, it is necessary todetermine whether wastewater can be reused as rawwater. Supernatant concentrations of samples A2, A4and A6 exceeded the allowable 25 mg/L. Hence,supernatants mixed with raw water are below rawwater quality standard will be discussed later.

Assessing treatment efficiency

Figure 8 shows the variation in treatment efficiencyfor SS by each treatment process for the foursampling days. The treatment efficiency for SS is48.35–99.68%, a satisfactory rate for SS removal.

Figure 9 shows the variation in treatment efficien-cy for NH3–N by each treatment process for the foursampling days. The treatment efficiency for NH3–N

Fig. 9 The variation intreatment efficiency forNH3–N by each treatmentprocess for the four sam-pling days

Fig. 10 The variation intreatment efficiency forTOC by each treatmentprocess for the four sam-pling days

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is −41.41–46.43%. The removal efficiency of thewastewater treatment units decreased as the concen-tration of NH3–N increased. Therefore, the ozonationtreatment step in wastewater treatment oxidizes andremoves NH3–N, improving the follow-up treatmentefficiency.

Figure 10 shows the variation in treatment effi-ciency for TOC by each treatment process for the foursampling days. The treatment efficiency for TOC is−91.24–65.74%. For all four samplings, the treatmentefficiency for TOC removal is limited. The ozonationtreatment unit added to wastewater treatment reducesthe concentration of TOC.

Figure 11 shows the variation in treatment effi-ciency for turbidity by each treatment process for thefour sampling days. The treatment efficiency forturbidity is 24.15–99.36%. The treatment efficiencyfor turbidity removal is obvious except wastewaterbasin. Figure 12 shows the variation in treatmentefficiency for COD by each treatment process for thefour sampling days. The treatment efficiency of CODremoval during the four samplings is inconsistent.

Feasibility of wastewater reuse

According to maximum limits for raw water qualitystandards to discuss feasibility of wastewater reused.Analytical results for CCLWTP were compared withthose at the Fongshan traditional water chlorinationtreatment plant, which is another major supplier ofdrinking water to Kaohsiung city.

The concentrations of NH3–N in supernatant mixedwith raw water were 0.12–0.14 mg/L, which is lowerthan the 1.08–1.10 mg/L at the Fongshan traditionalwater chlorination treatment plant. The mixed concen-trations of NH3–N were less than the 1 mg/L of thewater quality standard for drinking water.

The concentrations of TOC in supernatant mixedwith raw water were 1.22–1.47 mg/L, which is lowerthan the 2.71–2.88 mg/L at the Fongshan traditionalwater chlorination treatment plant. The mixed con-centrations of TOC were less than the 4 mg/L of thewater quality standard for drinking water.

The concentrations of COD in supernatant mixedwith raw water were 8.63–10.98 mg/L, which is lower

Fig. 12 The variation intreatment efficiency forCOD by each treatmentprocess for the four sam-pling days

Fig. 11 The variation intreatment efficiency for tur-bidity by each treatmentprocess for the four sam-pling days

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than the 12.06–13.63 mg/L at the Fongshan tradition-al water chlorination treatment plant. The mixedconcentrations of COD were far less than the25 mg/L of the water quality standard for drinkingwater. Therefore, the reuse of wastewater is feasible.

Conclusions and suggestions

1. The treatment efficiency of SS and turbidity are48.35–99.68% and 24.15–99.36%, respectively.Therefore, the wastewater treatment units confirmthat the SS and turbidity removal are significantthroughout the wastewater processes.

2. The removal efficiency of NH3–N, TOC andCOD are limited by wastewater treatment pro-cesses. Because of wastewater treatment units isused as a substitute for sedimentation and sludgedewatering of wastewater recycling. Thus, waste-water treatment units were not effective inreducing NH3–N, TOC and COD. Therefore, theozonation treatment step in wastewater treatmentoxidizes and removes NH3–N, TOC and CODimproving the follow-up treatment efficiency.

3. During wastewater treatment processes, NH3–N,TOC and COD of mixing supernatant and rawwater is regulated raw water quality standards.Therefore, supernatant reuse is feasible andworkable during wastewater processes at theCCLWTP.

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