Optimising the preparation of activated carbon from digested sewage sludge and coconut husk.

Preparation of activated carbon from sewage sludge is a promising way to dispose of sewage sludge as well as to produce a low-cost adsorbent for pollutant removal. This research work aimed to optimise the condition for activated carbon preparation from anaerobically digested sewage sludge with the additive coconut husk. The sewage sludge sample was mixed with the additive coconut husk. The preparation condition variables investigated involved the concentration of the ZnCl2 solutions, heating temperature, dwell time and heating rate in pyrolysis and the mixing ratio of coconut husk to sewage sludge. Surface area, pore size distribution, aqueous phenol adsorption capacity and the production yield of the final products were determined and compared. Experimental results revealed that low concentrations of ZnCl2 solution tended to improve the microporosity of the final product. Heating temperature had a considerable impact on the surface area, pore size distribution and phenol adsorption capacity of the final products, whereas dwell time and heating rate performed comparatively insignificantly. The effect of increasing the mixing ratio of coconut husk to sewage sludge was principally to increase the microporosity of the final products. The activated carbon with the highest BET surface area was produced with the activation of 5 M ZnCl2 solution and, thereafter, pyrolysis at a heating temperature of 500 degrees C for 2 h with a heating rate of 10 degrees C/min. The mixing ratio of 1:4 in terms of coconut husk to sewage sludge based on their dried weights was found to be most cost effective.     

Techno-economic assessment of coconut biodiesel as a potential alternative fuel for compression ignition engines

Over the past years, there were dramatic improvements in identifying and assessing various feedstocks for the production of biodiesel fuels. To promote a particular feedstock as a renewable source of energy, it is important to analyze their energy, economic, and engine performance characteristics. The current work attempts to evaluate the net energy and economic indices for both fossil diesel and coconut-blended diesel (B20) considering the diesel consumption by the Indian railways. Further, we present the experimental results of a multi-cylinder diesel engine operated with neat coconut biodiesel (B100) and fossil diesel at various load and speed conditions. The engine experiments reveal that the coconut biodiesel exhibits leaner combustion and shorter ignition delay than fossil diesel. Lower amount of carbon monoxide, hydrocarbon, and smoke emission is observed in the case of coconut biodiesel, with higher levels of nitric oxide (14%) and fuel consumption than diesel. The coefficient of variation in indicated mean effective pressure is within the range of better driveability zone for both the fuels at all test conditions. Overall the engine performance, emission and combustion results with neat coconut biodiesel are favorable with a penalty in NO emission at high load conditions. The techno-economical study highlights higher production cost per liter of B20 than the cost of fossil diesel. However, the net energy ratio (NER) for B20 is 1.021, favoring higher output than diesel and thus lowers the dependency on crude oil.

     

Clean Technology for Treatment of Photographic Wastes and Silver Recovery

Photographic industry which is extensively used by the medical and film industries heavily depend on silver. The industry produce large quantities of wastewater containing high concentrations of silver compounds. Silver in all forms is regarded as toxic heavy metal and prohibited from discharging into public sewers. Also silver is an expensive commodity and recovery of silver from these wastewaters and recycling improves production cost. Thus makes the process an environmentally friendly operation. This paper gives an insight of an improved silver extraction process developed at the building department of Ngee Ann Polytechnic to effectively recover silver from photographic wastes and to treat the waste water to standards acceptable for disposal, by the Ministry of the Environment.     

Physical and chemical properties study of the activated carbon made from sewage sludge

Preparation of activated carbon from sewage sludge is a promising way to produce a useful adsorbent for pollutants removal as well as to dispose of sewage sludge. The objective of this study was to investigate the physical and chemical properties of the activated carbon made from sewage sludge so as to give a basic understanding of its structure. The activated carbon was prepared by activating anaerobically digested sewage sludge with 5 M ZnCl2 and thereafter pyrolyzing it at 500 degrees C for 2 h under nitrogen atmosphere. The properties investigated in the present study included its surface area and pore size distribution, its elemental composition and ash content, its surface chemistry structure and its surface physical morphology. Furthermore, its adsorption capacities for aqueous phenol and carbontetrachloride were examined. The results indicated that the activated carbon made from sewage sludge had remarkable micropore and mesopore surface areas and notable adsorption capacities for phenol and carbon-tetrachloride. In comparison with commercial activated carbons, it displayed distinctive physical and chemical properties.     

Application of Non-Conventional Filtration Technologies in Pollution Control

Deep bed filtration is becoming increasing important in wastewater treatment particularly in tertiary treatment as stringent water quality standards are emphasized. A detailed pilot-scale filtration study conducted with secondary effluent in a sewage treatment plant indicated that tertiary filtration with prior alum flocculation is capable of producing high quality effluent, particularly an effluent with phosphorus content of less than 0.3 mg/L (Vigneswaran and Ngo, 1994). Main problem of dual media filter has been rapid headloss development thus frequent backwash requirement. To overcome this problem two independent experimental studies were conducted: one with mobile bed filtration (MBF) and the other with combined downflow floating medium flocculator/prefilter (DFF) and coarse sand filter (CSF) system. A semi-pilot scale mobile bed filtration study conducted using oxidation pond effluent indicated that MBF with contact-flocculation arrangement was good in removing COD, T-P and turbidity with practically no headloss development. Sand of 0.6-0.84 nm dia and 50 cm depth was sufficient to reduce the COD and P from 55 mg/L and 4.5 mg/L to 15 mg/L and 0.5 mg/L respectively at a filtration rate of 7.5 m3/m2.h. Another experimental study with combined DFF-CSF system indicated that the DFF with in-line flocculation addition is a good pretreatment unit to reduce the phosphate upto 80-90%. The fact that DFF resulted in uniform filterable-flocs, it can also successfully be used as a static flocculator/prefilter unit. The introduction of DFF on top of a coarse sand filter increased the filter run time and removal efficiency (more 90-95% of NH3-N and T-P removal respectively). Most importantly, the backwashing of this system was achieved with small quantity of water at low backwash indicated that the removal efficiency was also superior with this system apart from the major advantage of low operating cost.     

Media Factors Affecting the Performance of Upflow Anaerobic Packed-Bed Reactors

Numerous studies have demonstrated that anaerobic packed-bed reactors (APBRs) containing media such as clay chip, small stone, and porous plastic modules are effective for treating a variety of organic wastes. Most of these studies focused on the treatment capabilities of the processes, yet little work was done about the factors affecting their designs and performance. This paper presents the results of a study on a laboratory scale APBR system examining the significance of media factors such as media roughness, specific surface area, porosity and pore size on treatment performance. The results showed that media roughness and pore size were more important over surface area on the reactor performance. Within the same media porosity, pore size plays a more significant role than surface area in term of COD removal efficiency. The study indicates that the ability of a support medium to retain biomass either as suspended growth entrapped in the void space, or by attachment to the media surface is a significant consideration in an upflow APBR. A testing protocol for measuring methane gas produced by the suspended biomass using a batch serum bottle technique was developed and presented in this study. The method involved measurements of hourly methane production and instantaneous TOC concentration from the suspended biological solids extracted from various reactor heights. The proportion of methane production associated with the suspended biomass was as high as 58% at organic loading rate of 16 g COD/L.day. The results suggest that media pore size and porosity play a significant role in the performance of upflow APBRs.     

A comparative study of anaerobically digested and undigested sewage sludges in preparation of activated carbons.

Disposal of sewage sludge is an increasingly expensive and environmentally sensitive problem throughout the world. Preparation of activated carbon from sewage sludge offers an attractive re-use alternative to the traditional disposal routes. The objective of this research work was to compare anaerobically digested sewage sludge (DS) and undigested sewage sludge (US) as source materials in the preparation of activated carbons. Prior to the preparation the properties of the two types of sewage sludges were determined and compared. Subsequently the sludge samples were activated with 5 M ZnCl2 solution and thereafter pyrolysed at heating temperature of 650 degrees C for 2 h with the heating rate of 15 degrees C/min under a nitrogen atmosphere. The produced activated carbons were characterised by surface area and porosity analysis, CHN elemental composition and ash contents determination, and aqueous phase phenol adsorption tests. The results indicate that in comparison with the DS, the US had a higher carbon content and lower ash content, and accordingly yielded a better activated carbon with a higher BET surface area, pore volume, carbon content and phenol adsorption capacity.     

The Nature of Biodegradation of Vegetation in Mangrove Ecosystem

Experiments were carried out in situ and in the laboratory for 45 and 90 day periods respectively to study the nature and process of biodegradation of leaves/cladodes of 9 species of halophytes with special reference to mangrove vegetation. The leaching rate of chlorophylls a, b, bacteriochlorophylls a, c, d, phaeopigments, organic carbon and micronutrients such as Zinc, Copper. Iron and Manganese were studied at different intervals (10, 30, 90 days) and in varying salinity media (0.30, 16.60, 33.30%_°S). The organisms involved in fragmentation, decomposition and biodeterioration have been listed. Total litter production in the wooded mangrove area was 7,457.07 tonnes/year (leaf litter alone 5,834.4 tonnes/year). The mangroves export substantial organic material to the neighbouring estuarine and sea waters and the values were estimated at 261 tonnes C/year and 1,566 tonnes C/year respectively. Only 783 tonnes C/year were utilised and retained for use within the mangrove ecosystem.     

Conditioning of Oily Sludge with Alum

Sludge dewatering is a process whereby water is removed from sludge so as to reduce its volume and alter its physical state from semisolid to damp solid. This physical change reduces the volume of sludge considerably and therefore the cost of disposal. The degree of reduction of sludge volume or dewatering is a function of the characteristics of sludge and the type of dewatering device. Sludge characteristics such as specific resistance, capillary suction time(CST), filter yield and solid content can be determined in the laboratories. Knowledge of these characteristics with different dosages of sludge conditioners helps in effective dewatering and sludge handling operations.Oil and grease have an affinity for suspended solids. Experiences show that the presence of oil in wastewater results in poor substrate utilization causing less aerobic treatment and subsequently hinder settling and dewatering processes. Sludge samples with different oil contents varying from 1.8% to 8.0% by weight have been examined in the laboratory to find out their specific resistance, capillary suction time and filter yield. Alum was used as a conditioner. Different dosages of alum varying from 2% to 12% by weight were used to determine the optimum chemical dosage for varying oil contents. Buchner funnel apparatus, filter leaf apparatus and capillary suction time test apparatus were used to determine the dewatering characteristics specific resistance, filter yield and capillary suction time respectively.Addition of alum decreases the specific resistances and capillary suction times of oily sludges rapidly upto 4% dosages. Alum dosages beyond 4% only increase the solids content in the sludge cake and increase the sludge volume to be handled. The total suspended solids of filtrate decrease with alum dosage. The correlations between dewatering characteristics were studied. A correlation between CST and specific resistance to filtration was established. CST can be measured easily and quickly in the laboratories. Using the CST and the correlation a quick prediction on dewaterability can be established.