Pakistan Textile Journal. October index

Generation and disposal of sludges from textile wastewater treatment units
by Mahmood Akbar

This article is based on various policies, practices, legislations and directives of US environmental Protection Agency (USEPA) Similar laws, however, exist with EPAs on all the developed countries but since USA have undertaken lead in this field hence USEPA has been mainly referred in this article. All mentioned documents are available on the internet. The idea of writing / compiling this article is to provide some information about various issues concerning generation, handling and disposal of sludges coming out from waste water treatment plants. Since issue of sludges being generated by the industrial and especially textile sector in Pakistan, is a burning topic regarding its hazardous and non-hazardous nature, hence an attempt has been made to provide know-how to all those who are operating wastewater treatment plants (WWTP).

More than 50 small and big capacity WWTP have been established so far in the country, but Pakistan Environmental Protection Agency (PEPA) have yet to guide the operators of WWTP plants regarding handling, storage and constituents of sludges which render sludge hazardous and or non-hazardous. In the absence of a clear directive, legislation (SRO etc), the exporters of textile products are at the mercy of foreign buyers who are dictating them to implement their respective country specific specifications on various environmental issues including those related to handling of sludges and their safe disposal.

Textile production involves a number of wet processes that may involve the use of solvents.(1) Emission of volatile organic compound (VOCS) mainly arise from textile finishing, dyeing process, and solvent use. VOC concentrations vary from 10mg carbon/m³ for thermosol process to 350 mg carbon/m³ for dyeing and condensation processes. Process waste water is a major source of pollutants. It is typically alkaline and has high biological oxygen demand (BOD) from 700-2000 mg/liters, and high chemical oxygen demand (COD) at approximately 2-5 times of BOD. Wastewater also contains solids, oils and possibly toxic organics including phenol from dyeing and finishing and hologenated organics from process such as bleaching. Dye wastewater are frequently highly coloured and contain heavy metal such as copper and chromium.

The initial treatment methods for wastewater were basically anaerobic processes where collected sewage was allowed to digest and stabilize essentially undisturbed. (2) However, towards the end of the 1800s several researchers, with the idea that aerobic treatment would avoid malodorous anaerobic conditions and undesirable results, began to explore the possibility of blowing air into sewage tanks. Over the course of the next three decades, experiments in aerobic treatment of wastewater lead to the conclusion that aerating wastewater in the presence of a suspended biomass (achieved through solids recycle) is a very effective method of treating wastewater to degrade the biological constituents in the wastewater. In 1914, this process was termed as “activated sludge process” and has since become the standard method for wastewater treatment. The activated sludge process is a biochemical type of reaction. It involves the mass transfer of oxygen from an oxygen containing gas into the wastewater and then the mixing and use of that dissolved oxygen to support the growth of aerobic microorganisms suspended in the wastewater. These microorganisms, known as the biomass, oxidize the organic materials in the wastewater in different ways to eliminate the biochemical oxygen demand of the wastewater.

Oxygen necessary for the process is provided by air or oxygen. The aeration is usually increased by the use of gas-liquid contacting devices such as diffusers, surface aerators, and sparging mixing impellers. Usually the process operates in a continuous-flow mode, but it can also be operated as a semi-batch process. The contents of the aeration tank are termed as mixed liquor and contain micro organisms (living and dead), soluble contaminants, and inert, biodegradable, and non-biodegradable suspended and colloidal matter. The particulate solid portion of the mixed liquor is termed “mixed liquor suspended solids“ (MLSS).

After a sufficient residence time for the biological reactions to occur (usually a number of hours), the mixed liquor is typically transferred to a separate gravity settling basin (clarifier) to allow separation of the MLSS from the treated wastewater. Settled MLSS is then recycled to the aeration tank as recycled sludge to maintain a sufficiently concentrated microbiological population for rapid degradation of the influent wastewater constituents. Because there is usually a significant net positive production of biological MLSS (the rate of cell production exceeds the rate of cell destruction) an increasing inventory of sludge solids builds up in the system and the excess activated sludge is to be discarded or “wasted” from the process continuously or periodically. The final result of the activated sludge process is two separate streams: the treated effluent wastewater and the excess waste activated sludge stream. The effluent is a liquid/water stream very low in suspended solids content and soluble pollutants which are sometimes again treated to further treatment options for the activated sludge. Treated wastewater effluent include nutrient (nitrogen and phosphorus) removal processes and sterilization through ozonation, etc.

The waste activated sludge stream from the secondary wastewater treatment process is very high in solids content - about 0.5-1.5 wt % total solid prior to thickening and about 2-3 wt % total solids after thickening. The waste activated sludge is often combined with sludge from a primary clarifier operating in front of the activated sludge process. It is highly desirable to process the total primary plus waste activated sludge in such a manner that it can be readily and economically disposed of without creating further pollution of the ecosphere.

Sludge is the residual semi-solid material left from industrial, water treatment, or WWTP processes.(3) When fresh sewage or wastewater is added to a settling tank, approximately 50% of the suspended solid matter settles out in about an hour and a half. This collection of solids is known as raw sludge or primary solids and is said to be "fresh" before anaerobic processes become active. Once anaerobic bacteria take over, the sludge will be effected by bacteria in a short time and must be removed from the sedimentation tank before this happens. Sludge, the semisolid residual, also include an unpredictable mix of heavy metals, synthetic chemicals, radioactive waste, medicines, and so on. Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from the raw water.

Excess solids from biological processes such as activated sludge can be referred to as sludge, although more often called "biosolids," a public relations term that is increasingly used by water professionals in the United States. The sludge is transformed into biosolids using a number of complex treatments such as digestion, thickening, dewatering, drying, and lime stabilization.

The basic aim of all wastewater sludge treatment processes is to economically and efficiently reduce and stabilize waste sludge solids. In conventional practice final sludge disposal is commonly carried out by incineration, land filling or land spreading. In many instances, land disposal is employed and is particularly attractive due to minimal long-term environmental effects and is highly advantageous in contributing to reconditioning of the soil. However, the use of land spreading as a final sludge disposal method may require a well stabilized and pasteurized end product, so that the concentration of pathogenic organisms in the sludge is sufficiently low to avoid it becoming a health hazard. Also, the sludge should be adequately stabilized to prevent further degradation in the environment.

Traditionally, three distinct processes have been widely utilized for treating wastewater sludges, which are non-hazardous in nature,(2) (i) Oxidation Ponds, (ii) Anaerobic Digestion, and (iii) Aerobic Digestion.

Oxidation ponds are generally employed in the form of comparatively shallow excavated earthen basins which extend over a large area of land and retain wastewater prior to its final disposal. Such ponds permit the biological oxidation of organic material by natural or artificially accelerated transfer of oxygen to the pond water from the ambient air. During the bio-oxidation process, the solids in the wastewater are biologically degraded to some extent and ultimately settle to the bottom of the pond, where they may become anaerobic and are further stabilized. Periodically, the oxidation pond must be drained and the settled sludge dredged out to renew the volumetric capacity of the pond for further wastewater sludge treatment. The withdrawn sludge is utilized for example as landfill. Oxidation ponds thus represent a functionally simple system for wastewater sludge treatment. The use of oxidation ponds, however, has limited utility, since their operation requires sizable land areas.

Anaerobic digestion has generally been the most extensively used wastewater sludge digestion process for stabilizing concentrated organic solids, which are removed from primary settling tanks, biological filters and activated sludge plants. In common practice, the combined excess waste sludge is accumulated in large covered digesters where the sludge is mixed and naturally fermented anaerobically for about 30 days. The major reasons for the widespread use of anaerobic sludge digestion are that this method is capable of stabilizing large volumes of dilute organic slurries, results in significant biological solids (biomass) reduction and stabilization, produces a relatively easily dewaterable final sludge for ultimate disposal, is a net producer of methane gas, and is potentially capable of producing a pasteurized sludge under the right conditions. Anaerobic digestion is carried out in large scale tanks which are more or less thoroughly mixed, either by mechanical means or by the recirculation of compressed digester gas.

As an alternative to the foregoing anaerobic methods, biodegradable wastewater sludge can be digested aerobically. Air and to a much lesser extent high purity oxygen has been employed in practice as the source of oxygen for this purpose. It is also well known that aerobic digestion proceeds more rapidly at elevated temperatures. As the temperature rises above 40°C., the population of mesophilic microorganisms declines and thermophilic forms increase. The temperature range of about 50-70°C. is often referred to as the thermophilic range where thermophilic bacteria predominate and where most mesophils are extinct. Thermophilic digestion can also effectively reduce or eliminate pathogenic bacteria in the sludge (pasteurize the sludge), thereby avoiding the potential health hazard associated with its ultimate disposal.

However, more recently air sludge digestion systems operating in the thermophilic temperature range have become more common by employing such techniques as covered and highly insulated tanks, external heat sources and heat exchange equipment to minimize both the gas phase and liquid phase heat losses from the aerobic digester contents. Air contains only 21% oxygen and only about 10-20% of the oxygen content thereof is dissolved and available to the bacteria in an air aerated aerobic sludge digestion system.

Hazardous waste sludge is a waste with properties that make it dangerous or potentially harmful to human health or environment.(3)

The range universe of hazardous waste is large and diverse. Hazardous waste can be liquids, solids, contaminated gases, or sludges. They can be byproducts of manufacturing processes or simply discarded commercial products, like cleaning fluids and pesticides listed wastes. US EPA determined that by definition some specific wastes are hazardous. These waste are categorized into three types which are as follows:

This list identifies wastes from common manufacturing and industrial processes. Since the processes producing these waste can occur in different sectors of industry, the F-listed wastes are known as waste from non-specific sources.

This list includes certain waste from specific industries, such as petroleum refining or pesticide manufacturing. Certain sludges and wastewaters from treatment and production processes in these industries are examples of specific wastes.

These lists include specific commercial chemical products in an unused form. Some pesticide and some pharmaceutical products become hazardous when discarded. A waste which does meet any of the listing above may still be considered hazardous waste with exhibits of four characteristic such as ignitability, corrosivity, reactivity and toxicity.

This is a sensitive issue which is of due concern to textile wastewater plant operators in Pakistan, as PEPA have not issued any SRO on this subject.(5) Paragraph 1 Solid Waste Disposal Act, applies to the following hazardous wastes/sludges listed or identified under Section 3001.

a. Liquid hazardous wastes, including free liquids associated with any solid or sludge containing free cyanides at concentration greater than or equal to 1000 mg/l.

b. Liquid hazardous wastes, including free liquid associated with any solid or sludge containing the following metal (or elements or compound of these metals (or elements) at concentrations greater than or equal to those specified below:

d. Liquid hazardous wastes containing polychlorinted biphenyles at concentrations greater than or equal to 50 ppm.

e. Hazardous waste containing hologenated organic compounds in total concentrations greater than or equal to 1000 mg/kg.

It is therefore, inferred that values of various metal constituents, if are below the above specified limits, the nature of sludge shall be non-hazardous.

From the chemical compositions of inputs being used for bleaching, dyeing, printing and finishing in textile processing units in Pakistan and elsewhere, it is also hoped that sludges generated by their WWTP shall all be non-hazardous in nature.

The disposal of hazardous wastes (HWs) can be effected in many different ways, some of them are discussed as under:(3)

Recycling: Many HWs can be recycled into new products. Examples might include lead-acid batteries or electronic circuit boards where the heavy metals can be recovered and used in new products.

Neutralization: Some HWs can be processed so that the hazardous component of the waste is eliminated, making it a non-hazardous waste. An example of this might include a corrosive acid that is neutralized with a basic substance so that it is no-longer corrosive (see acid-base reactions).

Waste-to-energy: A HW may be “destroyed” for example by incinerating it at a high temperature in specially designed units. Flammable wastes can sometimes be burned as energy sources. For example many cement kilns burn HWs like used oils or solvents.

A landfill, also known as a dump or pit, is a site for the disposal of waste material. Historically, landfills have been the most common method of organized waste disposal and remain so in many countries of the world. Landfills are of the following types:

Inert landfills only manage concrete, asphalt, masonry, ceramics glass, aluminum and stainless steel. These types of material do not pose any significant environmental risk.

Limited purpose landfills are performance based and include industrial waste, demolition waste, problem waste and wood waste. These landfills may sometimes include a liner to prevent the contamination by leachate migration downwards through the underlying geological formation, and a leachate collection system.

Leachate is the liquid that drains or 'leaches' from a landfill. The generation of leachate is caused principally by precipitation percolating through waste deposited in a landfill. The percolating water becomes contaminated and it then flows out of the waste material.

Industrial solid waste landfills are made as per design standard and have a composite liner (consisting of recompacted soil lined by a flexible membrane liner) and a leachate management system. The use of linings is now mandatory within both the United States and the European Union except where the waste is genuinely inert. In addition, most toxic and difficult materials are now specifically excluded from land-fill.

More modern landfills in the developed world have some form of membrane separating the waste from the surrounding ground and in such sites there is often a leachate collection series of pipes laid on the membrane to convey the leachate to a collection or treatment location.

The physical appearance of leachate when it emerges from a typical landfill site is of strong odors, yellow or orange coloured cloudy liquid. The smell is acidic and offensive and may be very pervasive because of hydrogen, nitrogen and sulfur rich organic species such as mercaptans. These risks of leachate generation are there and these can be mitigated by properly designed and engineered landfill sites that use impermeable liners made of geo-textiles or engineered clay.

A more significant risk may be the failure or abandonment of the leachate collection system. If a leachate collection system fails, leachate levels will slowly build in a site and may even over-top the containing membrane and flow out into the environment. Leachate is free to egress the waste directly into the groundwater. In such cases high concentrations of leachate are often found in nearby springs and flushes.

A HW may be sequestered in a HW landfill or permanent disposal facility.(3) “In terms of hazardous waste, a landfill is defined as a disposal facility or part of a facility where hazardous waste is placed in or on land and which is not a pile, a land treatment facility, a surface impoundment, an underground injection well, a salt dome formation, a salt bed formation, an underground mine, a cave, or a corrective action management unit.

The liquid waste, flammable waste, hospital and other clinical waste which is infectious are not accepted by a landfills.

In recent years some countries e.g. Germany, Austria, and Switzerland have banned the disposal of untreated wastes in landfills. In these countries, only ashes from incinerators are deposited in landfill.

A hazardous waste / sludge can also be dumped temporily in polyethylene bags on a concrete surface which is impervious to the ground below. It is generally covered with tarpaulins.

Authors note: Due to comprehensive nature of this topic, as elaborated in foregoing pages, the justice has not being done, as each individual topic can further be researched for additional details. It is also to be understood that PEPA have to frame their own standards limits for hazardous wastes disposal. These limits may not be in agreement with those of developed countries.

(1) Pollution Prevention and Abatement Handbook World Bank Group Effective July 1998.

(2) Continuous Multistage Thermophilic Aerobic and Aerobic-Anaerobic Sludge Treatment System USA Patent No 7329340.

(3) WikipediaR, the free encyclopedia (It is a registered trade mark of Wikipedia Foundation Inc. USA).

(5) US EPA Solid Waste Disposal Act December 31st 2002. Tittle II Solid Waste Pages 1-163 (Also known as Resource Conversation & Recovery Act RCRA).