Membrane and Adsorption Processes for Removing of Organics and Inorganics from Urban Wastewaters

Since in Kosovo there are still no water purification plants and untreated wastewaters are discharged in environment, in this paper we’ve studied methods for removing of different organic and inorganic pollutants from Kosovo urban wastewaters. For best results we’ve used two methods, reverse osmosis and adsorption. For reverse osmosis, all samples were pretreated with coagulant (FeSO4) and flocculant (CaO) and then treated with reverse osmosis membranes. For adsorption, we used Kosovo coal ash and bentonite, both natural and low cost adsorbents. The analysis of experimental results shows that removing of organic and inorganic pollutants by reverse osmosis was very effective and removed from 93-98% of organics and almost 100% of heavy metal ions. Efficiency of coal ash in removing organics from natural waters was from 88-95%, while the efficiency of bentonite was 77-88%, while removing of heavy metal ions by coal ash was from 79-100% and by bentonite was from 50-92% respectively. keywords: coal ash, reverse osmosis, membranes, bentonite, adsorbtion.

Lignite type coal in Kosovo was extensively used in electricity power plants.Unfortunately the exploitation of lignite was done without respect to environmental capacity of the country and almost has created an environmental bomb for population.Uncontrolled burning of lignite in power plants each year left in Kosovo environment about 2 million tons of fly and bottom ashes, more than 100 thousand t of sulphur, of which more than 50% organic, over 11 t of arsenic, more than 2 t of beryllium, 1 t of cadmium, 351t of nickel, 492 t of titanium, 191 t of manganese, 0.48 t vanadium, etc.This uncontrolled burning of lignite in Kosovo power plants, jeopardize both our public health and our environment 8,9 .
Since coal is main natural resource of Kosovo, we've used it in reverse osmosis for modification of membranes and in adsorption for removing of different pollutants from natural and wastewaters.Since Kunst and Sourirajan 10 produced productive asymmetric cellulose acetate membranes for a low pressure reverse osmosis application many investigations have been initiated for further development of these membranes.Exploitation of new membrane materials, modification of existing membrane materials by using chemical or physical methods such as plasma modification radiation induced grafting, surface chemistry reaction, surface coat and blend method have been frequently used.Especially, the blend method has become widespread and efficient to prepare a good membrane 11 .In order to obtain membranes with specific properties, additional additives can be added to the casting solution.Coal was used as new material to prepare heterogeneous reverse osmosis membranes 12 .It was found that aryldiazonium salts at low concentration in their organic or aqueous solution are materials able to be electrochemically reduced on carbon semiconductors [13][14][15] or conductors 16 .Therefore, this coal was modified with 4-nitrobenzene diazonium salt in acetonitrile or hydrochloric acid.The membranes from cellulose acetate and coal modification show more productivity in comparison than that non modified coal 17 .The modified coal with 4-nitrobenzene diazonium salt shows encouraging results for preparation of heterogeneous reverse osmosis membranes.
Reverse osmosis membrane can be well protected by pretreatment, as it is a significant determinant for employing water reuse plants in remote areas 18 .Currently, reverse osmosis feed pretreatment is mainly used to reduce suspended and colloidal material in the feed water.Conventional pretreatment includes disinfection, flocculation/ coagulation and filtration processes, including microfiltration or ultra filtration 19 .In the last decades, on many occasions, the membrane processes looks more and more promising as separation processes in industry and waste water treatment.
Among various water purification and recycling technologies, adsorption is a fast, inexpensive and universal method.Activated carbon is considered as universal adsorbent for the removal of diverse types of pollutants from water.The use of various low-cost adsorbents (from organic precursors like fertilizer industry wastes, fly ash, biomass, peat, fruit stones etc. and inorganic precursors including red mud, clays, blast zeolites etc.,) as substitutes for the costly activated carbon, have been extensively studied and have led to the rapid growth of research interests in this field [20][21][22] .Heavy metals pose a significant threat to environment and public health because of their toxicity, accumulation in the food chain and persistence in nature.Adsorption technology is one of the best water treatment technologies and has been used extensively for the uptake of toxic metal ions from ground, surface and wastewater.
Continuously increasing demand for raw materials and the limited availability of natural resources, as well as extensively used lignite type coal for producing energy gave rise to the investigation of possible reuse of fly ash and bentonite for extracting valuable trace elements or as adsorption material for heavy metals as pollutant in water streams [23][24][25][26] .Bentonite, another natural occurring material shows a wide range of industrial applications.It's abundance in most countries and its low cost makes it a suitable adsorbent for the removal of many pollutants 27 .For removal of organics and inorganics from water samples we've used coal ash as adsorbent, which meet dual goals of disposal and treatment, and as comparison we've used bentonite.Although both used adsorbents are alum silicates, they differ in their composition.There is predominance of alkaline components (CaO and MgO) in Kosovo coal ash and in bentonite there is predominance of acid components (SiO 2 , Al 2 O 3 ).

Coal modification
The coal specimens were treated with boiling water (353K) under stirring conditions.The residual coal after filtering was dried at 378 K to constant weight.Typically 8 g of this coal was dispersed in 100 cm 3 phosphoric acid containing 15 mM of N 2 C 6 H 4 BF 4 .The solution was stirred at 278 K in controlled water bath for 5h.The reaction mixture was filtered with a 589 Blue ribbon and then the modified coal was washed by successive aliquots of water.Finally the modified coal was dried at 378 K to the constant weight, ground and sieved.The coal fractions of sieve size of ~ 170 mesh were used in this study.

Film casting details
The film casting details for the investigated membranes are casting solution composition cellulose acetate (E 398-3) 10 wt%, modified coal with aryldiazonium salt 15 wt%, with corresponding amounts of acetone 61.3 wt%, water 12.25 wt%, and magnesium perchlorate 1.45 wt% at temperature of casting solution and casting atmosphere 297K and ambient air (relative humidity 60%).The films were cast on a clean glass plate (22x38 cm) with the use of a metal cylinder with uplifted edges to obtain the required film thickness (0.12mm).The glass plate was kept at the same temperature as the casting solution.The casting solution temperature and the external conditions of solvent evaporation time (0s) during film casting were the same for all series of films studied.The cast solution was immediately dipped into a gelatin bath consisting ice cold water (273K).The duration of the film setting in ice cold water was 1h.Before the reverse osmosis experiment, the membranes were preshrunk under water at different temperatures (the temperature of water bath was controlled to within ± 0.5K) and initially each film was subjected to pure water pressure treatment for 1h at 20% higher pressure than that to be used in reverse osmosis run.The surface area of the membranes investigated was 11.92 cm 2 .
Permeate flow rate, referring to the membrane-permeated solution corrected to 298K, and the rejection factor R, defined as: Cp and Cf -permeate and feed concentrations (g/ dm 3 ).

Pretreatment step
First the urban waste waters were treated by FeSO 4 and CaO in proportion 0.05:0.125g/dm 3 .This step was used as pretreatment method.After that these waste waters are treated with reverse osmosis membranes.After pretreatment the pH was increase till 10.7, so we used some drops H 2 SO 4 to decrease pH to 7, because the membranes degrade in this pH.

Reverse osmosis apparatus
The membranes, five samples of each type, were tested in a laboratory reverse osmosis apparatus illustrated schematically in Fig. 1 Adsorption As adsorbents we used coal ash from Kosovo thermo power plants (Kosova A and Kosova B) and bentonite.They were dried at 105ºC and calcinated at 800°C.Physical-chemical characteristics of adsorbents were determined with standard analytical methods (Table 3).1g of coal ash/bentonite was added to a certain volume of water sample (100 cm 3 ).Solution was continuously mixed with magnetic stirrer for 30 minutes.After completion of the reaction time, the mixture was filtered and analyzed.
Samples: S1 -urban waste waters of capital city of Kosovo, Pristina, S2 -urban waste waters of Prizren city, S3 -urban waste waters of Mitrovica city and S4 -urban waste waters of Ferizaj city.

RESULTS ANd dISCUSSION
Extensively used lignite type coal for producing energy, abundance of bentonite in nature and also their low cost makes them suitable adsorbents for the removal of many pollutants.In terms of physical-chemical properties, chemical composition of ash varies greatly, depending on the type of coal and its origin.Coal ash from our lignite type coal has specific inorganic composition.The results that were achieved in determining chemical composition of Kosovo coal ash and bentonite are shown in Table 1.
From the features shown in Table 1, it is expected that metal ions in water samples, will mostly be absorbed by silicon oxides, aluminum oxides or iron oxides, or by influence of combination of these oxides.It is assumed that the predominance of alkaline components (CaO and MgO) in Kosovo coal ash, will also affect the precipitation of some metal ions.
Comparative analysis of the two materials used as natural adsorbents, show the differences in their composition.Although both used adsorbents are alumosilikates, bentonite differs from the domination of acidic constituents (80%) to those alkaline (20%), while in coal ash dominates alkaline constituents (80%) to those acidic (20%) From the features shown in Table 2, we can see that these waters have pH 7.93-8.94and high electrical conductivity and TDS.From data for COD, BOD and TOC it can be seen that these waters are polluted.Experience from membrane waste water treatment has demonstrated that key design parameters must be followed to prevent rapid membrane fouling, and thus reduce height system maintenance costs and significant downtime.Chemical pretreatment process is one of the most popular methods for waste water pretreatment due to its low cost and simplicity.Pretreatment objectives, is to make the waste water to the reverse osmosis compatible with the membrane.Pretreatment is required to increase the efficiency and life expectancy of the membrane elements by minimizing fouling, scaling and degradation of the membrane.Inadequate pretreatment often necessitates frequent cleaning to restore product flux and rejection.This will result in excessive chemical cleaning costs, and in severe cases will result in permanent loss of performance, membrane degradation and therefore shorter membrane life.So, we used coagulant (FeSO 4 ) and flocculant (CaO) to pretreate water samples (S 1 , S 2 , S 3 , and S 4 ), see Table 2, 3 and Figure 1, 2.
From Table 2 and Figure 2 it can be seen the effect of chemical pretreatment with FeSO 4 /CaO in reduction of COD, Pt, NO 3 -by 53.4%, 58.9%, 60.1%,As we see from the data this step is very important.After the pretreatment samples were treated with reverse osmosis (RO) membranes.RO membranes can remove dissolved solids which can't be removed by chemical and biological or other conventional municipal treatment process.Same tables and figures show the results of RO membranes.From these data we can see that RO membranes have shown significant reduction of: TDS (95-97%), COD (93-98%), BOD (96-97%), TOC (97%), EC (95-97%).The flux of these membranes was 4.9x10 -2 m 3 /h/m 2 .
Reduction of TSS plays a significant role in modern wastewater treatment, since the SS serves as an adsorbent for heavy metals and polychlorinated biphenyls (PCBs).Results from Table 2 show that removal of TSS by coal ash was very effective from 73-100% and by bentonite from 61-80%.The presence of excess nitrogen and its compounds has a negative impact on the environment.Nitrogenous compounds play an important role in water pollution, therefore, the control of them in water has vital importance.Efficiency of removal of nitrate ion and ammonium ion from the treated water samples was high.Removal of nitrate ion by coal ash was 100%, with bentonite 95-100%, while removal of ammonium ion by coal ash was 87-95% and by bentonite 71-93% respectively.High levels of organic pollutants into river water affect the suitability of river water for human use and it stimulates the growth of algae and aquatic plants.Various treatment methodologies have been used for the removal of organic pollutants but most of them are complicated and time-consuming, except for adsorption methods,   Unt -untreated sample; RO -reverse osmosis; CA -coal ash; BE -bentonite; '! -after treatment DO was higher than instruments detection limit.The removal of total phosphorus by adsorption is simple and convenient.Results from Figure 2 show the removal of total phosphorus by coal ash was from 87-96% while with bentonite was 76-89%.

Sample
Table 2 shows that pH before and after treatment with coal ash are very different.pH values revolve primarily in line with the guiding values for waters discharged into natural ecosystems, before treatment they range from 7.93-8.94,after treatment with coal ash the pH values were increased up to 14, which is expected due to the composition of the ash, and after treatment with bentonite the pH values were increased from 9.44 -10.8 respectively.As for conductivity values before treatment they vary from 568-798 µS/cm and after treatment with coal ash the EC values were increased from 1175-2030 µS/cm, while after treatment with bentonite EC values were 623-840 µS/cm.From the features shown in Table 3 and Figure 4 it can be seen the effect of chemical pretreatment with FeSO 4 /CaO in removal of heavy metal ions Fe 2+, Pb 2+ and Cd 2+ by 57.5%, 61.7%, 71.4%.Same tables and figures show the results of RO membranes.From these data we can see that RO membranes have shown almost complete removal of heavy metals: Fe 2+ (90-100%), Ni 2+ (85-96%), Cr 3+ (83-96%), zn 2+ (97-100%), Mn 2+ (94-98%), Cu 2+ (96-100%), Pb 2+ (94-100%) and Cd 2+ (71-100%).These data indicates that resulting membranes are readily available and effective for treatment of municipal waste waters.The development of these membranes is of great interest for wide variety of low pressure reverse osmosis membranes especially for waste water treatment.
From Table 3 and Figure 5 it can be seen that both used absorbent showed very good sorption properties.Percentage of removal of heavy metal ions with coal ash was from 85-100% while with bentonite was from 50-92%.Increasing pH during adsorption indicates that the high efficiency of Kosovo coal ash in removing of metal ions can be attributed to two processes, the adsorption and precipitation process as a result of hydrolysis of the alkaline oxides in sodium and magnesium hydroxides which help precipitation of certain metal ions.The adsorption process is a result of characteristic structure of our coal, its high porosity and large surface area.

CONCLUSION
Based on the results that were achieved we can conclude that success of membrane processes depends on proper pretreatment, chemical control and RO membranes that are resistant to fouling.Data from this paper shows that these membranes are readily available and effective for treatment of municipal waste waters.The development of these membranes is of great interest for wide variety of low pressure reverse osmosis membranes especially for waste water treatment.Kosovo coal ash and bentonite show good sorption capacity toward heavy metal ions and other pollutants with high sorption rates of over 90% of the total adsorption which can be obtained in thirty minutes of contact time.

Fig. 4 :Fig. 5 :
Fig. 4: Percentage of removal of heavy metal ions after pretratment with FeSO 4 /CaO and after treatment with RO membrane

dm 3 mg/dm 3 mg/dm 3 mg/dm 3 mg/dm 3 mg/dm 3 mg/dm 3 mg/dm 3
which are low cost and easy to use.Coal ash and bentonite were very effective adsorbents for removal of organics.Removal of COD by coal ash was from 83-94% while with bentonite was 73-88%.