Design, Development and Economic Evaluation of Small for Stp

8/18/2019 Design, Development and Economic Evaluation of Small for Stp

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DESIGN, DEVELOPMENT AND ECONOMIC EVALUATION OF SMALL AND DECENTRALIZED WASTE WATER TREATMENT SYSTEM

Sivacoumar, R*., Latha, K. and Jayabalou, R.Scientist, National Environmental Engineering Research Institute (NEERI),CSIR Madras Complex, Taramani, Chennai-600 113, India.*Email: [email protected]

Abstract: Rapid urbanization, population growth and limited land availability increased the global issue of watercrises and wastewater disposal method. In the present study, Fluidized Aerobic Biological treatment (FAB) systembased sewage treatment plant was designed scientif ically and installed in the 50 sites of 7 tsunami affected districtsof Tamil Nadu, India using modular design principle. FAB is an advanced technology which is a combination of attached and suspended growth process and fluidized condition is maintained in the reactor by providing diffusedaeration from the bottom of the reactor. The installed STP consists of high rate FAB reactor and tube settler formsheart of the treatment system to remove organic and suspended solids. The STPs were designed in 8 modules of capacity 55, 75, 100, 150, 200, 250, 300, 450 KLD based on standard design criteria and engineering consideration of the

sites. The total area requirement varied 0.64-0.746 m2/KLD which is 6 times lesser than conventional methods.Economic evaluation was done for all the installed modules in 50 sites so as to access and validate the cost effectivenessof the FAB reactor. The cost and economic analysis indicated that capital cost, recurring cost and total cost of treatment per KLD varied Rs.13, 620-23,980, Rs.384-880 and Rs.14, 094-24,575 respectively. The civil, electro-mechanicaland operation and maintenance cost contributes to 50-72%, 26-50% & 2-4% of the total treatment cost respectively.The performance evaluation of installed STP at these 50 sites indicated the removal efficiencies of TSS, BOD & COD varied 83-85%, 92-93% and 74-80% respectively. A computer aided design package was designed and developed forcost estimation and performance evaluation of the mathematical model formulated to predict the STP cost usingstatistical analysis. The models formulated indicated that the accuracy was good (99%) and the error was less forelectro-mechanical cost model (0.017 lakhs) followed by civil cost model (0.529 lakhs).

Key words: Fixed-film biological reactor, Moving bed biological reactor, Conventional treatment methods,Performance evaluation, Modular design, Lamellar flow tube settler, Tube settler, Design package, Mathematical

cost model

Received on: 10 October 2013, accepted on: 12 December 2013

INTRODUCTION

Safe water supply and hygienic sanitationfacilities are the two basic essential amenitiesthe community needs on a top priority forhealthy living. Wastewater generation in Indiais increased from 7,000 MLD in 1978 to 35,558MLD in 2008 in class I cities (CPCB, 2009). Asof 2003, it was estimated that only 30% of India’s

wastewater was treated and the rest 70% f indsits way to local rivers and streams contributingto water pollution. In addition to this,occurrence of natural disasters also contributesto sanitation problem and environmentalpollution. Providing environmentally safesanitation to millions of people in India is asignificant challenge, especially in the world’ssecond most populated country.

Tsunami a natural disaster hit the southernIndian Ocean on December 24, 2004, affectingthe entire coastal area of Tamil Nadu (1076 km). About 154,000 houses were damaged of which80% belonged to fishers (ADB, 2005). In orderto overcome the sanitation issue and to safeguard the coastal area from environmental

pollution, Government of Tamil Naduscientifically designed a cost effective andsustainable Fluidized Biological Reactor (FAB)based Decentralized Wastewater Treatment(DWWT) system in 8 modules of capacity 55,75, 100, 150, 200, 250, 300, 450 m3/day (KLD) totreat the sewage generated from the tsunamiaffected 50 sites covering the 7 districts of Nagapattinam, Cuddalore, Villupuram,

Journal of Aquatic Biology and Fisheries Vol. 2/2014/ pp. 704to 716


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Kancheepuram, Tirunelveli, Kanniyakumari andToothukudi (Fig. 1). The STPs were designedto meet the effluent discharge standards toeffectively utilize the treated effluent thereby reducing the water crises.

There are many conventional wastewatertreatment methods but they require relatively large capital investment, large area, powerintensive and require a lot of monitoring (Helen,2008). Since, the sites were along the coastal areathere was a need for compact, low cost andefficient treatment method which could beachieved through decentralized approach definedas collection, treatment, and disposal/reuse of wastewater at or near the point of waste generation.In addition, decentralized treatment system andmodular installation reduces the sewagetransportation cost significantly (NEERI, 2008).

The objective of the study is to design ascientific, compact, cost effective and efficientSTP to treat the sewage generated from housingclusters constructed for tsunami affected areasto provide a well planned wastewatermanagement system along the coastal area withan option to reuse the treated effluent. Thestudy also includes formulation and evaluation

of a mathematical model using statisticalanalysis for predicting civil, electro-mechanicaland operation & maintenance cost for FAB baseddecentralized STP to validate the cost effectivesof the treatment method.

METHODOLOGY Selection of STP capacity

The prime importance in the design of a sewagetreatment plant is to estimate the capacity of theSTP to be designed. The volume of sewagegenerated for all the 50 sites was quantifieddirectly by a flow meter. The capacity of the STP was estimated based on the quantity of sewagegenerated.

Wastewater Characteristics

Assessing sewage characteristic is essential forthe choice of treatment method, extent of treatment, beneficial use of the wastewater andcost of treatment required (Ross, 2000; Rao,2007). Samples of raw sewage were collectedfrom all the 50 sites for determination of pH,TSS, TDS, TS, BOD & COD as per standardmethods (APHA, 2005). Sampling was carriedout in 3 stages consisting of one week for each

Fig. 1. Location of STP sites


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stage. First stage of sampling was done for a weekfollowed by second stage of sampling after amonth interval and third stage of sampling after2 months.

Design of STP

Once the sewage characteristic is assessedsuitable cost effective and economical treatmentmethod must be chosen for designing a STP.Since all the 50 sites in the study weredistributed along the coastal area, it was decidedto go in for DWWT method. DWWT method ispreferred as compared to conventionalcentralized treatment system, since it enhancessewage treatment near the points of generation,thereby avoiding the sewage transportation cost(Lens, 2001).

Nagapattinam district which includes 30 sites was divided into 3 clusters and each cluster wasprovided with one STP. The cluster installation was done to min im ize th e se wag etransportation and maintenance cost. For allthe other 6 districts each village was provided with individual STP and they were designedin modular basis since it is cost effective andself contained. Modular installations arerelatively compact and transportable units andare ideal for use in remote location and requireless installation. Most importantly modulardesign was done to optimize the organic and

hydraulic loading rate of the treatment system.In addition, the prime advantage of modulardesign is that it can be easily added to theexisting module at the time of increase in thesewage generation volume so as to achieve themaximum design eff iciency in terms of organicand hydraulic loading rate.

Design of Fluidized Aerobic BiologicalReactor

The raw sewage generated from thehouseholds were processed through

preliminary treatments and pumped intoFABR (aeration tank), using submersiblesewage pumps, wherein the aerobic microbesattached to the media in the reactor utilizesthe biodegradable organic pollutant inpresence of oxygen there by reducing BOD/COD. The free, molecular oxygen requiredis provided through air grid supplied by airblower.

The dead biomass along with treated wastewaterflowed by gravity into tube settler tank, whereinsolid-liquid separation was achieved. Tubesettlers are high rate settlers of the inclinedsurface or shallow settling type. The settlers useinclined tubes to divide the depth into shallower

sections. Thus, the depth of all particles (andtherefore settling time) was significantly reduced. These clarifiers provide a large surfacearea reducing clarif ier size. No wind effect exists,and the flow was laminar. Tubes are circular,square, hexagonal, or any other geometric shapeand were installed in an inclined position withinthe basin. Tubes were commonly inclined steeply (550) to horizontal and fabricated in modules.

The solids separated were pumped to the sludgedrying bed for further volume reduction. Thetreated supernatant dosed with sodium

hypochlorite through and was allowed to reactin chlorine contact/filter feed tank prior tofurther polishing. The treated wastewater thenpumped with filter feed pump through dualmedia f ilter & activated carbon filter prior tosuitable reuse like gardening and toilet f lushing.The dead biomass was separated and transferredperiodically into the sludge drying bed whereinmass reduction occurs. The dried sludge wasfurther removed periodically and used as afertilizer for plants.

Design Criteria used for STP Design

Design criteria are guideline values used fordesigning new wastewater treatment facilities.Choosing the design criteria depends on theengineering conditions like topography of thearea, hydraulic head, ground water depth and itsseasonal variations, soil bearing capacity, type of strata expected to meet in the construction andthese data are obtained from field studies. Thetreatment units were designed based on thestandard design criteria according to the fieldconditions of the sites (NEERI, 2008).

Computer aided design for STP A computer aided design for STP was developedfor treatment plant design, cost estimation anddetailed statistical evaluation of the modeldeveloped. It comprises of design package in excelplatform for sizing the different units of STP.Design criteria, raw sewage characteristic,sewage volume, topography of the area wereused as the input data.


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Table 1. Quantity of sewage generated and their corresponding module capacity


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Table 2. Design criteria used for STP design

Design of STP

FAB based modular STP was designed based onstandard design criteria listed in Table 2. Theraw sewage generated from each village wasprocessed through preliminary treatmentfollowed by biological and tertiary treatment(Fig. 2). The dimensions of different units of STP including FAB are tabulated in Table 3.

The installed FAB was rectangular shaped withan average height of 4.5 m. The reactor was filled

with polypropelene carrier media occupying 30%of the total tank volume. The carrier media used

offered an effective surface area ranging 200-500m2/m3 for different shaped media. The reactors were designed for 9-12 hrs HRT and f luidizedcondition was maintained by providing diffusedaeration at the rate of 15 m/s thereby enhancingeffective oxygen transfer efficiency by allowingthe sewage in full contact with the media up on which the microbes are immobilized.


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During f luidization operation, the carrier mediaexpands to accommodate microbial growth.The higher the surface area for the biomass togrow higher will be the concentration of activebio-mass per unit volume of the reactor. Returnsludge which is indispensable for the activatedsludge process was not required enablingextremely easy maintenance control.

The total area required for FAB based STPincluding all preliminary units varied 6.40-7.46

m2

/KLD for 55-450 KLD, with land requirementof 0.7 m2 per KLD. On an average, FAB reactoralone occupies an area of 0.087 m 2/ KLD and it varied 0.075-0.095 m2/KLD. This contributes toan average of 12.6% of the total area ranging11%-13%.The high rate tube settler occupies anarea of 0.043-0.058 m2/KLD which constitutes7-8% of the total area required. This proves thatFAB based STP requires 6 times lesser land thanconventional treatment methods.

Performance evaluation of constructed STP

The removal efficiency of the FAB reactor interms of TSS varied 83-85%, whereas for BOD92-93% and COD 74-80% respectively. High rateof treatment efficiency within a short HRT wasdue to high surface area provided to themicrobes, high organic loading rate and thediffused aeration provided through bottom of the STP. As compared to conventional methodsFAB’s efficiency is high and consistent

irrespective of the climatic conditions since thesystem is compact and highly controlled.

Cost estimation

Cost estimation indicated the capital cost variedRs. 1,174,360-Rs.7,046,950 and the total treatmentcost varied Rs. 12,22,870-72,20,080 (Table 4). Theoperation & maintenance cost varied Rs. 48,500-173,130 per annum. The increase in the capacity of STP corresponds to a decrease in capital cost,annual O&M cost and total treatment cost per

m3 of sewage because of the larger STP capacity.The civil, electro-mechanical and operation &maintenance cost contributes to 50-72%, 26-50%& 2-4% of the total treatment cost for the modulesof 55-450 KLD.

Mathematical model

The mathematical model formulated for costestimation as per the procedure outlined insection 2.9 for civil units, electro-mechanical andoperation & maintenance cost is given below.

Plant civil cost model

Plant civil cost model was constructed withmodular STP capacity in x axis andcorresponding actual civil cost in y axis as shownin Fig. 3.

y = 0.176 x 0.893

where x denotes the capacity of the STP in KLDand y denotes the civil cost in lakhs.

Fig. 2. Schematic diagram for FAB based STP


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o m p u t e r a i d e d d e s i g n d e t a i l s o f S

T P


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C o s t e s t i m a

t i o n o f t h e S T P u s i n g c o m p u t e r a

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Plant electro-mechanical cost model

Plant electro-mechanical cost model wasconstructed with STP capacity in x axis andcorresponding actual electro-mechanical cost in y axis as shown in Fig. 3.

y = 0.363x0.644

where x denotes the capacity of the STP in KLDand y denotes the electro-mechanical cost inlakhs.

Plant operation and maintenance costmodel

Plant operation & maintenance cost model wasconstructed using with STP capacity in x axisand corresponding O&M cost in y axis as shownin Fig. 3.

y = 0.096 x 0.558

where x denotes the capacity of the STP in KLDand y denotes O&M cost in lakhs.

Plant total cost model

Plant total cost model was constructed usingmodular STP capacity in x axis andcorresponding total cost in y axis as shown inFig. 3.

y = 0.448 x 0.801

where x denotes the capacity of the STP in KLDand y denotes the plant total cost in lakhs.

Model performance evaluation

Model performance evaluation was carried outusing actual cost observed in the field and thecorresponding predicted cost. The summary of the statistical analysis of the mathematicalmodel showed that, the difference in observedand predicted mean was least for O & M costmodel (0.02 lakhs) followed by electro-mechanical cost model (0.05 lakhs) and civil

cost model (0.14 lakhs). The models formulatedindicated that the accuracy was good (99%)and the error was less for electro-mechanicalcost model (0.017 lakhs) followed by civil costmodel (0.529 lakhs).

CONCLUSIONS

FAB a hybrid reactor offers the followingadvantage compared to conventional STP:

Consistent and efficient removal of organic and inorganic pollutantsimultaneously within short HRT in onesingle reaction tank there by making itmore advantageous by occupying 6times less space

High treatment eff iciency viz., TSSremoval varied 83-85%, BOD removal92-93% and COD removal 74-80%

Return sludge and circulation of nitrifying liquid is not required,enabling extremely easy maintenancecontrol

Low operating cost as the tube settleremploys no moving parts

Low installation cost as it is preassembled, light weight, skid mountedrequiring minimum foundation

construction They can operate at high hydraulic and

organic loading rate The computer aided design package

developed for FAB sizing makes it easy for computing the dimensions of thetreatment units and cost estimation

The mathematical model performanceevaluation indicated that the accuracy of the mathematical model formulatedis 99% and hence could be used inpredicting the cost for FAB based STP

Hence, with all the above mentioned advantage,FAB proves to be an eff icient, economical andsustainable sewage treatment method suitablefor urban and rural areas.

ACKNOWLEDGEMENTS

The authors are grateful to the Director,National Environmental Engineering ResearchInstitute, Nagpur, India, for the facilities madeavailable for this work. They are also grateful tothe Revenue Administration, DisasterManagement and Mitigation Department,

Chennai for sponsoring the project

REFERENCES

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Duggal, K.N. and Chand, S. 2007. Elements of environmental engineering.

Guleda, O.E. and Ibrahim, D. 2006. Cost analysisof alternative methods for wastewaterhandling in small communities.Environmental Management, 79: 357-363.

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Lens, P., Zeeman, G. and Letting, G. 2001.Decentralized sanitation and reuse.

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NEERI. 2008. Basic design of sewage treatmentplant for tsunami affected areas in 7 districtsof Tamil Nadu. National EnvironmentalEngineering and Research Institute, Nagpur.

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Design, Development and Economic Evaluation of Small for Stp