التجربة العملية لاستخدام مجلطات مثل الالوم وكلوريد الحديديك وكبريتات الحديدوز فى ازالة المواد الرغوية العالقة واللون وانخفاض مستوى الاكسجين الكيماوى فى مياه الصرف الصناعى

The use of Alum, Ferric chloride and Ferrous sulphate as coagulants in removing suspended
solids, colour and COD from semi-aerobic leachate at controlled pH

BY

GENERAL.DR

BAHAA BADR

TECHNOLAB EL-BAHAA GROUP

Abstract

Suspended solids, colour and COD are amongst the main form of pollutants in leachate.

Application of physical or biological process alone is normally not effective to remove these constituents, especially for leachate with lower BOD/COD ratio.

The main objective of this research is to investigate the efficiency of coagulation and flocculation processes for removing suspended solids, colour and COD

Twelve months characterization of leachate indicated that it was stable with yearly average BOD/COD ratio of 0.15 and biologically difficult for further degradation.

Particle size analysis of raw leachate indicated that its d50 was 11.676 µm.

Three types of coagulants were examined using standard jar test apparatus, i.e., aluminum sulphate (alum), ferric chloride (FeCl3) and ferrous sulphate (FeSO4).

The effects of agitation speed, settling time, pH, coagulant dosages and temperature were exermined.

At 300 rpm of rapid mixing and 50 rpm of slow mixing and 60 minutes settling time, higher removals of suspended solids (over 95%), colour (90%) and COD (43%) were achieved at pH 4 and 12.

FeCl3 was found to be superior compared with other coagulants.

At pH 4 and 12, fair removal of suspended solids was observed at reasonably lower amount of coagulant, i.e., 600 mg/L. However, about 2500 mg/L of coagulant was required to achieve good removals at pH 6.

Better removals were achieved at higher temperature.

The d50 of sludge after coagulation at pH 4 and 2500 mg/L FeCl3 was 60.163 µm which indicated that the particles had been removed effectively from the leachate.

The results indicated that coagulation and flocculation processes had contributed bigger roles in the integrated treatment system.

1.0 Introduction

One of the most important aspects related to siting, planning, design, operation and long-term management of Municipal Solid Waste (MSW) landfill is the management of leachate.

Leachate from MSW landfill sites are often characterized as heavily polluted wastewater.

Leachate is a complex organic liquid formed primarily by the percolation of completed site).

If it was not treated and safely disposed, landfill leachate could be a potential source of surface and groundwater contamination, as it may percolate through precipitation water through open landfill or through the cap of the
completed site

Leachate may contain large amount of organic contaminants which can be measured as chemical oxygen demand (COD) and biological oxygen demand (BOD), ammonia, halogenated hydrocarbons, suspended solid, significant concentration of heavy metals and inorganic salts 0).

If it was not treated and safely disposed, landfill leachate could be a potential source of surface and groundwater contamination, as it may percolate through
soils and subsoils, causing pollution to receiving waters).

The specific composition of leachate influences its relative treatability and depends to a large extent on the contamination to be removed

In general, the treatment of landfill leachate often involves a combination of various techniques.

Several processes derived from water and wastewater treatment have been applied for the treatment of leachate.

These include aerobic and anaerobic biological treatments, photo-oxidation and membrane processes, chemical oxidation and precipitation, activated carbon and adsorption and coagulation-flocculation

Biological processes are quite effective for younger leachates (containing mainly volatile fatty acids), but less efficient for the treatment of stabilized leachate

However, leachate with low organic content is best treated with physical/chemical process

Chemical precipitation using lime indicated that between 70 and 90% removal of color, turbidity, suspended matter and dispersed oil could be achieved

Therefore, a combination of physical-chemical and biological methods is often required for the efficient treatment of leachate

Coagulation and flocculation is widely used in water and wastewater treatment and these techniques form an important step in the treatment process

Coagulation process is effective for removing high concentration organic pollutants
heavy metals and some anions

Aluminum and iron coagulants have been widely used as coagulants for the removal of humic substances from water

Chemical coagulants can destabilize colloidal particles by four distinct mechanisms: double layer compression; charge neutralization; enmeshment in a precipitate and inter-particle bridging

Different coagulants provide different degrees of destabilization.

The higher the valence of the counter-ion, the more will the destabilizing effect and the less amount of dose required for coagulation.

Alum, ferrous sulphate, ferric chloro-sulphate and ferric chloride were commonly used

we reported that iron salts seem to be more efficient than aluminum.

we reported that the addition of flocculants together with coagulants may enhance the flocs-settling rate.

Additionally we reported that nonionic, cationic or anionic polyelectrolyte could be used as coagulant aids to increase flocs settling rate, without really improving the turbidity removal efficiency.

However, limited information exists on the efficiency of the coagulation-flocculation process, when applied for the removal of suspended solids, colour and COD from semi-aerobic landfill leachate.

A semi-aerobic landfill is a system where semi-aerobic condition is maintained in the waste body through a convection process.

It involves the decomposition of organic matter inside the landfill and causes an increase in temperature.

The difference in temperature between the inside and outside of the landfill generates a heat convection current in the landfill through the leachate pipe

It was found that the leachate from semi-aerobic system has slightly lower organic contaminants compared with an anaerobic landfill in terms of BOD, COD, and other
constituents

The total area of the landfill is 23.7 ha and equipped with a leachate collection pond but without other treatments.

This site is contained by a natural marine clay liner.

It was further upgraded to a sanitary landfill level III by employing controlled tipping with leachate recirculation

This site receives 1,500 tons of solid waste daily ).

The objective of this study was to examine the efficiency of coagulation-flocculation processes for the removal of suspended solids, colour and COD from a semi-aerobic landfill leachate using three types of coagulants, i.e., aluminum sulphate (alum), ferric chloride and ferrous sulphate.

The experiments involved with the determination of the most appropriate coagulant types and dosages, the examination of the effect of pH on the removal capacity and the identification of the optimum experimental conditions for the efficient application of these processes.

2.0 Materials and method

Leachate samples were collected . If tests could not be carried out on the same day, the samples were stored in a refrigerator and maintained at 4°C Coagulation and flocculation studies were performed in a standard jar-test apparatus

(Jar Tester Model CZ150) comprises of six paddle rotors (24.5mm x 63.5mm) and equipped with 6 beakers of 1 L volume.

Leachate samples were removed from the refrigerator and conditioned for about 3

hour under ambient temperature.

Samples were thoroughly agitated for re-suspension of settled solids before any tests were conducted.

Chemical reagents used as coagulants are commercially available from R&M Marketing, UK which include aluminum sulphate [Al2(SO4)3.18H2O], ferric chloride (FeCl3) and ferrous sulphate [FeSO4].

Stock solutions were prepared by dissolving 50g of these salts in 1 L of deionized water.

Each beaker used for testing was filled with 500mL of sample.

The influences of agitation speed (rapid and slow mixing) and settling time were examined in the pre-experiments at pH 6 and at a coagulant dosage of 500 mg/L.

The influence of agitation speed was determined in the subsequent experiment by varying the rapid mixing from 250 to 350 rpm and slow mixing from 20 to 70 rpm. Variations of settling times were made between 10 and 210 seconds.

In the pre-experiments suspended solids were used as controlled parameter.

In the main experiments, constant dosage of coagulants (1000 mg/L) was used to determine the optimum pH.

In order to determine the optimum dosage of coagulant, initial pH of 4, 6 and 12 were used.

The initial rapid mixing for all experiments was taken as 60 seconds (range between 30-300 seconds, Ramirez & Velasquez, 2004; Aquilar et al., 2005) and for slow mixing at 19 minutes (range between 5-55 seconds, Tatsi, et.al, 2003; Aquilar et al., 2005).

After settling (duration, pre-determined in the pre-experiment), about 50 mL of supernatant was withdrawn using plastic syringe from the point located about 2 cm below liquid level for the determination of pH, suspended solids, colour and COD. Analyses were made in triplicates.

Experiments were conducted at 25°C and 10°C.

Selected flocs formed after settling were sampled for particle size distribution using Malvern 2000-S Mastersizer. In this study, the analysis was conducted for raw leachate and coagulated leachate (sludge)

at pH 4, using 2500 mg/L FeCl3 as coagulant.

The pH was measured by pH meter (CyberScan 20).

Colour measurements were reported as true colour (filtered using 0.45µm filter paper) assayed at 455 nm using DR 2000 HACH spectrophotometer that was adapted from Standard Methods for the Examination of Water and Wastewater (APHA, 1995) (Method No 2120C).

The result is reported in Platinum-cobalt (PtCo), the unit of colour being produced by 1 mg platinum/L in the form of the chloroplatinate ion.

The effect of filtration on colour removal was corrected by means of a control sample. COD were determined in accordance with Method 5220D (closed reflux, colourimetric method).

Removal efficiency of suspended solids, colour and COD was obtained using the following equation:
(Eqn. 1)

Where Ci and Cf are the initial and final concentrations of leachate, respectively.

2.0 Results and discussion

The characteristics of raw leachate are presented in Table 1. In terms of pH, the leachate could be categorized as alkaline.

In terms of COD, Table 1 suggests that the leachate has moderate concentration of organic matter.

The annual average BOD/COD ratio was 0.15 which reflects the presence of high concentrations of recalcitrant organic matter which is stable and biologically difficult for further degradation.

The permissible limits for Standard A and B are 50 mg/L and 100 mg/L respectively.

The presence of high concentrations of colour was contributed by the presence of dissolved organics.

These organic compounds may present in the form of recalcitrant material of humic acids
Humic substances are natural organic matters made up of complex structures of polymerized organic acids, carboxylic acids, and carbohydrates

The level of NH3-N was very high (average value 970 mg/L) which was the result of slow leaching and the release of soluble nitrogen from solid waste in landfills, which may last for several decades

This implies that there is a need to treat the leachate before it can be discharged. Integrated treatment systems consisting of biological, physical and chemical treatments are often necessary to treat this type of leachate.

However, the focus of this paper is on the coagulation and flocculation processes.

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