The following article will guide you about how to treat wastewater from pesticides industry.
Existing Treatment Facilities:
The flow diagrams of wastewater treatment adopted in some of the industries are shown in Figs. 16.18 through 16.24.
Pesticides industries have adopted generally two schemes of pollution control as given below:
ADVERTISEMENTS:
(i) The first scheme is evaporation system which further includes the following steps:
(a) Segregation of production waste from utility and sanitation waste.
(b) Evaporation of the segregated waste in suitably designed evaporation pans (solar evaporation or forced evaporation).
ADVERTISEMENTS:
(c) Incineration of the concentrated waste.
Some pesticides industries, having effluents which are not easily biodegradable and also toxic, are adopting solar evaporation of effluents for the following reasons:
(a) Ease in management of effluents by solar evaporation.
(b) To overcome high cost involved in the construction and operation of a full-fledged effluent treatment plant.
ADVERTISEMENTS:
(c) To avoid treatment of effluents to the levels of Minimal National Standards (MINAS) and to overcome effluent disposal problems.
(ii) The second schema is detoxification followed by biological treatment (conventional activated sludge). In this system, the wastewater from various streams is detoxified, wherever it is toxic and is then combined and treated in an effluent treatment plant which includes primary and secondary treatment.
The primary treatment includes oil and grease removal, equalisation, neutralisation, coagulation and settling. The secondary treatment includes aerobic biological treatment with aeration tanks, secondary clarifler and sludge drying beds.
Observations on Existing Treatment Systems:
The problems associated with the present wastewater treatment options adopted by the industries are given below:
(A) Solar Evaporation System:
(i) The solar evaporation system has limitations based on quantity of effluent and climatic conditions at the site. The effluent in the evaporation tanks if not a leachate-proof holding arrangement, may cause pollution by virtue of percolation, overflow and evaporation of toxic volatile matter into the, atmosphere.
The rate of evaporation generally varies from 2.5 mm/sq. m/day in monsoon season to 12.0 mm sq. m. /day in summer. The predominant climatic conditions i.e. rainfall and rate of evaporation in an area play a major role in limiting the quantity of wastewater that can be evaporated.
(ii) The rate of evaporation is low during rainy season and hence an industry will be left with excess quantity of wastewater, unless the evaporation system is well designed. This excess quantity needs to be managed through alternate arrangements or else it can be a potential source of pollution.
(iii) There are chances of contamination of groundwater where the groundwater table is high and the evaporation pond is permeable.
(iv) The toxic volatile gases from the effluents may cause air pollution during the process of evaporation and the pollutants such as oil and grease, organic solvents may reduce the rate of evaporation.
(B) Biological Treatment System:
(i) The characteristics and the quantity of wastewater in a pesticides industry are varying from product to product and also seasonally, daily and hourly, whereas the ASF (Activated Sludge Plant) type of biological treatment system needs a relatively uniform rate of organic loading and without shock loads due to toxicity, non-uniform flows etc.
(ii) The organic, compounds through biodegradable, have complex organic chains due to which the operation of a biological system needs skilled staff.
(iii) There cannot be a single generalised treatment solution for the wastes from the pesticides industries due to the varying processes of manufacture and the varying characteristics of waste.
(iv) The wastewaters, unless properly segregated, bear high dissolved solids thus affecting the biological treatment.
(v) Wherever the biological treatment is affected, the microbial growth is limited and hence sludge return rate is to be maintained at a higher level in order to maintain food to micro-organism (F/M) ratio. But in the process sludge gets aged sooner and thus poses problem for biological treatment.
(vi) Under extreme climatic conditions, biological treatment does not work efficiently as the microbes find difficulty in surviving.
(vii) Activated sludge plants are generally adopted for aerobic treatment. These are high energy intensive and need skilled manpower for operation. Also, for high temperature locations, oxygen requirement is high and hence energy consumption is high.
Thus, activated sludge plants have limitations as a technological option for aerobic treatment in India where temperature is high and skilled manpower is not provided for ETP operation.
(viii) In most cases it is observed that the treatability of the wastewater is not tested, operation of activated sludge plant not monitored and the continuous aeration not provided. This results in unsatisfactory performance. For optimal operation of activated sludge plants, skilled manpower, back-up technical infrastructure and uninterrupted power supply are needed.
Disposal of Wastewater:
From the point of wastewater discharge various industries it is observed that wastewater is disposed to:
(i) On land for irrigation (factory’s own land).
(ii) Public drain/sewer/nullah.
(iii) River/creek.
(iv) Sea.
(v) Solar evaporation tanks (within factory premises).
Choosing the mode of disposal for treated effluent is very important in view of the toxic nature of effluent. The level of treatment required is to be decided based on the actual quality of the receiving body. If a toxic effluent is let into a public sewer, it can kill the microbes which are essential for the biological treatment of the sewage.
Similarly, due to land application of effluent, there are possibilities of groundwater contamination due to percolation and water pollution due to surface run-off. Disposal of effluent in river and sea can kill fishes and disturb other bio-activity. In case, river water is used for drinking purposes immediately in the downstream of the disposal point, it poses threat.
Wastewater Management:
The conventionally adopted end-of-the-pipe treatment technology is not only ineffective but is also costly. Also, in view of the effluent disposal problems and the complexity of the pollution problem from the pesticides industries, it is required to manage the wastewater very effectively such that the waste is minimised, cost in treatment and risks due to pollution are lowered and the environment is protected.
Various aspects for an effective wastewater management are discussed below:
1. Location:
The environmental impact due to an industry not only depends on wastewater generations, emissions and solid waste release into environment, but also depends to a large extent on the quality of the environment at the location of the industry.
Before the advent of pollution control legislation, industries were located considering largely economic factors, such as availability of raw material, labour, market etc. which would maximise their profits.
But now, with increased concern for environmental protection, a need has arisen to consider environmental factors for locating an industry.
The impact on the receiving environment from the pesticides industry which handles hazardous and toxic chemicals and emits obnoxious gases and wastewater depends on:
(i) Nearness to residential settlement and size of the settlement.
(ii) Climate, soil type, groundwater and geology at the site.
(iii) Adjointing landuses – agricultural, industrial, economically unproductive areas etc.
(iv) Nearness to sensitive zone/areas.
(v) Infrastructure availability for wastewater discharge and hazardous/solid waste disposal.
(vi) Existing quality of air, surface water, groundwater etc. at the site.
Hence, it is very important to select a suitable site so that the impact on the receiving environment is the least.
2. Plant Layout:
Plant layout is to be well-designed for an effective drainage system, with ease in collection and disposal of wastewater and storm-water. It also helps in safety, eases access, makes various activities of storage, scarp yard etc. well-defined and reduces misuse of areas.
A well-laid out plant with well-defined areas, buffer zones, and plantation, display boards and with colour codes for various process lines and storage tanks also helps in building a good industrial culture which has far-reaching results in improving process efficiency and preventing pollution besides safety.
i. Environmental Audit:
The conventional end-of-the-pipe treatment technology is not only ineffective but also costly, sometimes imposing a very heavy burden on the industry. The growing environmental pollution and the complexity of this problem with increasing risks from the regulatory controls need an effective management tool so as to prevent pollution by minimising losses, through recycle/recovery and to make pollution control programmes cost-effective and feasible.
Environmental audit is an effective management tool in this direction for an industry. It also acts as ‘liaison’ with the Government and the public in view of the mandatory requirement to submit an environmental statement to the Government before the 30th day of September every year from 1993.
Environmental audit comprises a systematic, documented, periodic and objective evaluation of how well the management systems are performing with the aim of:
(i) Waste prevention and reduction.
(ii) Assessing compliance with regulatory requirements.
(iii) Facilitating control of environmental practices by a Company’s management.
(iv) Placing environmental information in the public domain.
The environmental audit helps in pollution control, improved production, safety and health and conservation of resources in an industry and as a whole helps in achieving sustainable development of the country. Environmental audit has far-reaching benefits to the industry, to the society and the nation at large.
Some of the benefits of environmental audit are:
(i) Helps in assessment of performance of process system and pollution control systems.
(ii) Serves as an eye-opener for waste minimisation.
(iii) Identifies areas of cost savings by reduction in losses and pollution load, and by recycle and recovery of wastes.
(iv) Increases environmental awareness.
(v) Helps in understanding ‘Technical Capabilities and Attitude’ of the organisational set-up.
(vi) Provides up-to-date environmental data-base for use in plant modification, emergencies etc.
(vii) Unravels surprises and hidden liabilities due to which regulatory risk and exposure to litigation can be reduced.
(viii) Ensures independent verification, identifies matters needing attention and provides timely warning to management on potential future problems.
(ix) Helps safeguard environment.
(x) Assists in compliance with laws and regulations, Company’s policy and environment standards etc.
The audit procedure broadly includes the following:
(i) Pre-audit activities.
(ii) Activities at the site.
(iii) Post-audit activities.
Reduction of Raw Material Losses:
(i) Keeping only appropriate inventory of raw materials to ensure minimum material handling losses, evaporation losses etc.
(ii) Adopting mechanical handling of materials with proper monitoring facilities so as to dose only the predetermined quantities as per norms prescribed.
(iii) Having plant layout properly made so as to minimise transfer distance of materials between storage and process or between unit operations.
(iv) Preventing cross-contamination due to usage of same storage tanks for different materials depending on the batch product. Separate storages are to be provided.
(v) Separating process lines for separate products or separate equipment for each unit operation to minimise losses due to residues in the equipment, which are usually washed out.
(vi) Providing storage tanks with proper dip arrangements for exhausts/vents and insulation provided so as to reduce evaporation losses.
(vii) Enclosing and covering material storage areas to keep them secured and reduce losses due to carry over by wind and rain.
(viii) Making enclosures to collect spills and overflows of materials at the material transfer and sampling points for reducing.
(ix) Having regular maintenance to check flange leaks, breaks/cracks, pump failures etc.
(x) Ensuring raw material purity.
(xi) Selecting proper type of minimise losses due to residues in containers (raw materials should be easy to handle).
(xii) Following good house-keeping practices.
(xiii) Fixing norms for performance of various process operations so that the material usage is minimised and hence the material losses are minimised.
ii. Recovery:
There is a great potential in pesticides industry, for recovery recycle/reuse of useful products which otherwise go waste. These recoveries result in waste minimisation.
Some of the recycle/reuse practices that can be easily adopted are:
(i) The solvents which are widely used in the manufacture of pesticides can be recovered and recycled/reused.
(ii) In most of the chemical reactions, there are more products formed than the required products. These unwanted products form wastes if left unrecovered. For example – methyl chloride is formed while manufacturing DDVP.
These unwanted products such as, methyl chloride, HCl, monochlorobenzene, dichloroethane, trimethyl phosphite ester, sodium sulphite, sulphur dioxide, sodium thiosulphate etc. can be recovered as valuable by-products by employing a suitable technique such as condensation, distillation, absorption etc.
(iii) Condensate water can be collected and recycled/reused.
(iv) Floor washing, at times, can be collected, treated if necessary and recycled/reused,
(v) Seal pot water, vacuum pump water and cooling water can be recycled and reused in operations, such as neutralisation, washing, etc.
Recycling and Reuse of Wastewater:
Recycling and reuse of wastewater within the industry would help in minimisation of fresh water requirement and simultaneous reduction in wastewater volume for final treatment and discharge thereby reducing the related costs.
From the investigation of material balance and unit operations of a particular process, it will be possible to characterise the waste and identify the possibilities of recycling or recovery of useful products.
The process of identification of recoverable matter and recycling of wastewater can be made effective through ‘environmental audit’. Recycling in industries will be a major step in reducing regulatory risks and public litigation due to wastewater discharge outside causing impact on environment.
The recycling options in an industry can be as follows:
(i) Treating some or all process wastewater to make it suitable for plant process, cooling water make-up, floor-wash etc.
(ii) Recirculating the same water within a unit operation several times before it becomes unfit.
(iii) Sequential use of effluent from one process with treatment, if necessary, as input into another.
(iv) Usage of wastewater from a process to a lesser duty usage where an inferior quality of water will do. For example, it can be used for coal ash quenching in boiler house, for making lime solution used for neutralisation in the ETP etc.
In-Plant Control:
Good-housekeeping is the least expensive means to reduce the overall burden on treatment and disposal. Loss of raw material, solvent and product can be restricted by installing monitoring devices and the process managers held responsible for spills and poor housekeeping practices.
Also the flow in various drains can be checked using flow indicators and flow meters and controlled accordingly. In some cases, wastewater generation can be substantially reduced by substitution of an organic solvent for water in the synthesis and separation operation of the process with subsequent solvent recovery.
The steam jet ejectors and barometric condensers can be replaced in some cases by vacuum pumps and surface condenser systems. Barometric condenser produces a high volume of dilute waste stream and poses problems by contaminating the plant effluent.
Methyl chloride gas, hydrogen chloride gas, hydrogen sulphate gas etc., which are generated in the manufacturing process of pesticides, may be recovered or useful by-products produced instead of scrubbing and discharging to ETP.
Waste segregation and treatment also form an important step in in-plant control of pollution. This aspect of waste segregation is elaborated elsewhere in the report.
Treatment Methods:
For arriving at the requirement of treatment, the sources of wastewater and its characteristics are to be identified. The amount of treatment to be provided depends on the standards to be achieved for recycle and reuse or disposal to water body, land etc.
The treatment technology that is generally available for removing various pollutants is briefed below:
Suspended solids are removed by screens, grit chambers, sedimentation, floatation, centrifugation and filtration. Effluents are neutralised using acid/alkali. Oil and grease is removed by gravity separation and air floatation.
For removing heavy metals, pH is adjusted so as to precipitate these metals for removal by coagulation, settling and filtration. For the treatment of biodegradable organic effluents, biological treatment is adopted where in micro-organisms degrade the waste in presence of oxygen (aerobic treatment) or in absence of oxygen (anaerobic treatment).
Aerobic treatment systems are—activated sludges process, extended aeration, aeration ponds, trickling filters and rotating horizontal tanks or vertical reactors (digesters). Oxidation of organic matter is also carried out chemically using chlorine, ozone, hydrogen peroxide etc. For removing dissolved solids, ion exchange resin and reverse osmosis techniques are used.
Adsorption by activated carbon, activated clay etc. is used to remove specific pollutants. Highly toxic effluents which cannot be economically treated are incinerated. In case the effluents are small in quantity, a pretreatment is given to detoxify the effluent and to remove volatile matter and are solar evaporated in lined tasks.
More details on treatment methods for some of the pesticides are given below:
1. Waste Segregation and Pretreatment:
Based on the characteristics of wastewater from each of the unit operations, the streams should be segregated and preliminary treatment provided to the extent required and feasible. This will provide to be economical and will make the final treatment effective.
Various streams generated from a pesticides industry are as follows:
(i) High acidic or alkaline stream.
(ii) Stream carrying oil and grease and floating organics.
(iii) Inorganic stream.
(iv) Organic stream.
(a) Easily biodegradable.
(b) Not easily biodegradable.
(c) Toxic.
(v) Cooling water bleed-off.
(vi) Boiler blow down.
(vii) Domestic wastewater from canteen, toilets, cloth-washings etc.
(viii) Streams of highly variable characteristics.
(ix) Miscellaneous—floor wash, spills, leaks, drum/vessel washing etc.
The pretreatment that may be employed to various streams are ‘neutralisation’ for highly acidic/alkali stream, ‘oil and grease trap’ for oil and floating organics bearing streams, ‘settling tank’ for stream bearing high suspended solids and ‘detoxification’ for toxic streams.
Separate equalisation for streams of highly variable characteristics can be effective in achieving overall treatment efficiency. The recoveries employed in various streams including that of inorganic/organic, form a part of pretreatment and help in reducing pollution load to a considerable extent.
The separation technologies may be employed for removal of organic and pesticidal chemicals from wastewater.
The destruction and detoxification technologies, that prove very effective as pretreatment or part of final treatment, in simplifying the complex and stable organic structures of (chemical) pollutants and in removing toxicity are:
(i) Chlorine dioxide oxidation.
(ii) Dye sensitised photo-oxidation.
(iii) Electro-chemical oxidation.
(iv) Flameless catalytic oxidation (low temperature vapour oxidation).
(v) High energy radiation.
(vi) Hydrogen peroxide oxidation.
(vii) Incineration/combustion process.
(viii) Micro-biological and other metabolic systems.
(ix) Ozonation.
(x) Ultraviolet light assisted ozonation.
(xi) Photo-decomposition – ultraviolet radiation.
(xii) Potassium permanganate oxidation.
(xiii) Pyrolysis.
(xiv) Wet catalytic oxidation.
2. Chemical Oxidation:
Oxidation by chemicals is an effective method for decomposition of many pesticides. The chemicals commonly used for oxidation are chlorine, chlorine dioxide, potassium permanganate, ozone, hydrogen peroxide and sodium peroxide.
In general, unsaturated organic compounds are more susceptible to ozone oxidation than saturated compounds. Parathion is oxidised by chlorine and ozone to a more toxic product known as paraoxone. Lindane in aqueous solution is readily degraded by ozonation and only partially affected by potassium permanganate.
Treatment of lindane with chlorination peroxides and aeration has no measurable effect. Aldrin is readily attacked by chlorination, KMNO4, ozone and aeration, but peroxides have no measurable effect.
Dieldrin concentration in aqueous solution is decreased by aeration. The organo-nitrogen pesticides, during manufacture, emanate cyanide bearing effluent which can be converted to lesser toxic cvanates using chlorine in alkaline medium.
3. Coagulation:
Suspended particulate matter, to some extent, can be removed using coagulant aids. DDT is easily removed by alum coagulation of doses typical of conventional water treatment. Efficiency of DDT removal is 98% for a 10 ppb load, and 97% for a 25 ppb load.
Lindane and parathion have low efficiency of removal by conventional alum coagulation and sand filtration. Aldrin, dieldrin, BHC and malathion have low efficiencies of removal by chemical coagulation.
4. Adsorption:
The adsorbents that prove effective in removing some of the pesticides are activated carbon, saturated clay systems, humic acid, organic acid, bentonite, aluminium silicates and hydrous magnesium aluminium silicate.
Activated carbon is effective in removal of pesticides, solvents, emulsifiers and odours. Aldrin, dieldrin, DDT, 2, 4-D, BHC, captan and 2, 4, 5-T have partial adsorption by clays.
5. Photochemical Degradation:
UV – ozonation is remarkably effective for degradation of halogenated hydrocarbon. The effect of UV radiation is isomerisation in case of dieldrin, reduction of chlorinated compounds, replacement of aromatic halogens by hydroxyl in case of chlorinated herbicides and elimination in case of carbamates.
6. Liquid-Liquid Extraction:
Solvents prove effective in extracting pesticides. Details of liquid-liquid extraction systems and percentage recoveries (1:1 solvent to aqueous phase volume ratio) are given in Table 16.6.
Pesticide is the general term for insecticides, acaricides, rodenticides, molluscicides, herbicides, fungicides and similarly active compounds. A wide range of compounds are used as pesticides and a correspondingly wide range of methods are used in the analysis of their residues. They can be broadly classified according to their general chemical nature into several principal types as shown in Table 16.6.
The use of organic pesticides in agricultural practice is now firmly established and is becoming more extensive. Despite the undoubted advantages in their usage, concern has been expressed at the potentially harmful effects of using pesticides which are stable and can accumulate in man and his environment.
The stages of analysis are as follows:
(i) The extraction of the residue from the simple matrix using an efficient and selective solvent.
(ii) The removal of interfering substances from the extract—usually referred to as the cleanup procedure. This often involves either chromatography or solvent partition.
(iii) The estimation of the quantity of pesticide residues, together with metabolites and breakdown products, in the cleaned-up extract. These are determined at very low levels (as low as 10–12 g) and to obtain this sensitivity, stringent requirements of selectivity are imposed on the selected method.
(iv) The confirmation of the presence of the residue by, for example – using a different method or the formation and identification of a derivative.
There are five basic methods of analysis:
(i) Functional group analysis – This method involves, for example, the colorimetric assay of a particular group or element in a compound. It does not give precise identification, requires thorough cleanup and consequently is not now used very frequently.
(ii) Biological test methods – These show the presence of toxically significant residues, by, for example, inhibition of the enzyme cholinesterase in animals by certain classes of pesticide. As it is possible to use these methods without clean-up they are useful as screening tests but are non-specific.
(iii) Chromatographic methods – These include thin-layer and gas-liquid chromatography which give separation and accurate identification and estimation of a wide range of residues.
(iv) Spectroscopic methods – These provide evidence of identity and can also in ideal conditions be used in quantitative analysis.
(v) Radiochemical methods – Included under this heading are neutron activation analysis, direct isotope dilution methods and the more sophisticated double isotope derivative analysis technique.
For all methods of analysis there is a detection limit below which it is impracticable to differentiate between the noise level of the background and the analytical signal. These levels range from 10–6 g for chemical methods to 10–14 g for radiochemical procedures. The importance of modern analytical instruments in pesticide analysis cannot be exaggerated.
7. Biological Degradation:
Some of the microbes that help in biological degradation are actinomycetes, filamentous fungi, soil micro-organisms, bacterial enzymes and streptomycetes. Parathion waste can be treated successfully biologically on combination with domestic waste.
2, 4-D wastewater can be treated biologically provided that dilution and nutrient requirements are satisfied. The bacterial enzymes activate the degradation of 2, 4-D, 2-CPA, 4-CPA and MCPA. Malathion degradation is highly effective in activated sludge process and degradation increases with increased rate of aeration.
Actinomycetes, filamental fungi and streptomycetes degrade PCNB (penta chlornitrobenzene) and several actinmycetes rechlornate DDT to DDD, but dieldrin is not easily degradable.
BHC (Lindane) is degraded more anaerobically rather than aerobically and similar is the case of other chlorinated pesticides viz. heptachlor, deldrin, etc. The extractable degradation wastes are degraded easily by anaerobic treatment rather than aerobic treatment.
The treatment technologies in India should take advantage of the high temperature and the long intensive periods of solar radiation. Trickling filters, oxidation and polishing ponds can be operated far more efficiently than the activated sludge plant, under the Indian conditions.
Energy consumption is less, operation simpler and the reliability greater in these systems. The biological treatment systems that may be suitable for India are anaerotic treatment, oxidation ponds and trickling filters.
8. Incineration:
Combustion or incineration is practised by many industries emanating toxic streams. Most organic compounds (liquid and gaseous) can be effectively destroyed by this method. The nitrogen compounds present in the wastes form nitro-oxides, sulphur-bearing compounds form sulphur dioxides and sulphuric acid and phosphorous-containing compounds produce P2O5 and phosphoric acid, on incineration. Similar other gaseous pollutants may be formed which are to be properly scrubbed. High ammonia level of wastewater causes problem in combustion.
Stripping is more effective in this case which would permit recovery of stripped ammonia. Compounds containing heavy metals including lead, mercury, arsenic etc. should be burnt only if proper air pollution control equipment is provided.
These metals can escape to atmosphere and travel great distances thereby affecting the environment. Compounds containing chlorine produce hydrochloric acid which can cause serious corrosion problems in the incinerator and pollution problems in the dispersion areas.
It is important to have a well-designed incinerator that provides required temperature, contact time and air pollution control equipment. In case of organo-phosphorous pesticides, if combustion is not complete, toxic isomers of original compound may be formed. Most pesticides are destroyed effectively at 800°C – 1000°C.
Effluent Treatment Plant:
Treatment Technology:
The treatment of liquid wastes can be achieved employing physical, chemical and biological methods. The common physical methods are flow equalisation, oil and grease removal, sedimentation with or without coagulation, sludge removal, filtration and adsorption.
Equalised and steady flows are very important for improved efficiency, reliability and control of treatment plant. Hence, flow equalisation is employed in the treatment of industrial wastewaters.
Equalisation dampens flow variation and helps achieving a constant or nearly constant flow rate to ETP and prevents variation in quality (characteristics) due to blending of various streams. The equalisation tank needs proper mixing and aeration.
Mixing is required to blend the contents of the tank and to prevent deposition of solids and aeration is needed to prevent the wastewater from becoming septic (anaerobic-devoid of oxygen).
Proper sedimentation, with the removal of suspended solids including colloids from the wastewater, helps in reduction of BOD, COD, oil and other contaminants. Usage of a coagulant helps in agglomeration of colloids and subsequent sedimentation.
The sedimentation system includes flash mixing. A rapid mix helps in rapid and uniform dispersion of the coagulant throughout the suspension and results in more efficient use of the coagulant.
Flocculation is a gentle mixing that provides increased opportunities of contact between particles in suspension, thereby forming bigger flocs. Sedimentation, flash mixing and flocculation may be carried out in individual units or in one combined unit such as ‘clariflocculator’. The settled sludge is to be periodically removed to a filter press or sludge drying bed.
The chemical treatment includes neutralisation, oxidation, detoxification etc. Neutralisation is required to maintain a pH of 6.5 – 8.5.
It is required for:
(i) Corrosion control.
(ii) Reaction control on many chemical treatment processes.
(iii) Protection of system organisms, if direct discharge is employed.
(iv) Protection of micro-organisms in biological treatment system.
Neutralisation is generally done using lime solution. Employing caustic soda entirely being costlier, lime may be used partly followed by automatic dosing of caustic. In manual control of pH, especially where lime is used, it is observed that either over-dosages or under-dosages are done. The quality of lime varies and so the dosage rate varies.
The usage of lime or caustic leads to huge quantities of sludge and the usage of caustic leads to increased TDS. Hence, the usage of chemicals for neutralisation has to be optimised. On-line pH metres and automatic pH adjustment may be employed.
Highly alkaline conditions can detoxify certain pesticidal compounds. Caustic soda is often used for pH adjustment so as to bring about hydrolysis of some chlorinated and organophosphate manufacturing wastes.
Biological treatment includes aerobic treatment and anaerobic treatment. For aerobic treatment, adequate food, oxygen, nutrients and proper pH and temperature are required. It may be designed to operate in a pH range of 6 to 8.
The biological population requires inorganic nutrients for cell synthesis, including nitrogen, phosphorous, iron, silica and others.
If these nutrients are lacking in the effluent, they should be added. Biodegradability is an important factor for determining the application of biological treatment to an effluent.
Almost every organic compound can be broken down biologically but some, such as tertiary aliphatic compounds, benzene, trichlorophenols, pentachlorophenol etc., pose problem and do not degrade easily.
Some of the factors which affect biodegradability are:
(i) Compounds in emulsified or chelated forms are not readily available to micro-organisms (e.g. DDT).
(ii) Large and complex structure of molecules often limits the rate at which the compound can be broken down (e.g., carbamates and carboxylic acid compounds).
(iii) Aliphatic (straight and cyclic) compounds are in general more degradable than aromatic compounds.
(iv) The substitution of elements other than carbon in the molecular chain often makes the compound more resistant. Esters and epoxides, salts etc. are more resistant than the base pesticidal compound.
(v) Halogen substitution to an aromatic compound renders it less degradable.
A detailed study of the waste properties and biodegradation tests are necessary before a biological treatment method is designed.
The biological methods used for pesticide containing wastewater include:
(i) Trickling fillers.
(ii) Activated sludge plants.
(iii) Aerated lagoons.
(iv) Stabilisation ponds.
The treatment system to be adopted depends mainly on the standards to be achieved, reliability of the treatment system and its cost. The cost factor in an industry is expressed in terms of the percentage ratio of annual burden for ETP (annual loan repayment on capital costs including civil, mechanical and electrical components plus running costs) to annual sales turnover.
This percentage is acceptable, in general to a pesticides industry upto about 5%. Regarding reliability, physico-chemical treatment systems with automation are more reliable than the manual handling systems and biological systems.
Treatment Options:
For the final treatment of wastewater from the pesticides industry, there are many options depending on the type of waste.
(i) Highly toxic streams are generally concentrated and limited in quantities and hence may not be feasible to treat to the required standards economically. Hence this waste may be incinerated.
The spills and leaks in process areas, may be wiped out using cotton, sawdust etc. instead of using water and then this material incinerated.
The incinerator should be designed for suitable detention time and temperature so as to destroy the waste and should be provided with suitable air pollution control equipment for the flue gas treatment.
(ii) The vessels/reactors in a formulation unit can be washed using the same solvent that is used in the formulation and the washings reused for this formulation.
(iii) The toxic wastewater, which is not easily biodegradable, may be treated physico-chemically instead of treating biologically. This treatment includes detoxification, oil separation, equalisation, clariflocculation, oxidation with H2O2/NaOCl/KMnO4 etc., neutralisation and clariflocculation.
Subsequently, depending on the mode of disposal, there are options for further treatment, including the following:
Pre-concentration Followed by Incineration – The wastewater may be subjected to evaporation in an impervious holding arrangement so as to reduce its quantity and then incinerated.
Solar/forced evaporation – Where the quantity of wastewater is small (say <10 kld) and the climatic conditions are favourable, solar evaporation may be adopted. Forced evaporation, in view of its recurring cost, may be employed as a stand-by arrangement to the solar evaporation system. The wastewater subjected to solar evaporation need not be treated to the level of MINAS but should follow the above guidelines.
Oxidation using H2O2/NaOCl etc. followed by activated carbon filtration and collection in a tank along with cooling water, boiler blow-down and other treated wastewater. The inorganic and high TDS bearing effluent, which can only be treated by reverse osmosis, ion-exchange method etc., which prove to be costly and uneconomical, may be solar evaporated.
(iv) The biodegradable waste may be treated by detoxification, oil separation, equalisation, clarifloculation, aeration-stage I, clarification, aeration-stage II, clarification and activated carbon filtration. The toilet and canteen waste may be added in the aeration tank-stage I.
Various important aspects to be considered for effective functioning of biological treatment system for wastewater from pesticides industry are:
(a) The detoxification of waste is essential as otherwise; this waste will kill the micro-organisms, thereby making treatment system defunct.
(b) The wastewater is to be well-equalised and flow to aeration tank kept constant.
(c) Addition of toilet/canteen waste to aeration tank would help in supplementing.
(d) Mixed liquor suspended solids (MLSS) in the aeration tank may be kept above 5000 mg/l. Microbes activity in the aeration tank must be regularly observed under microscope.
Whenever the microbes (MLSS) reduce in quantity due to reduced available food in the effluent or death caused by shock loads of effluent, additional nutrient in the form of N (nitrogen), P (phosphorous) and K (potassium) must be supplemented in the aeration tank.
Also, if required the activated sludge from the aeration tank- stage II may be added to multiply microbes so that the malfunctioning of aerobic treatment due to the death of microbes can be avoided.
(e) An operation schedule is to be maintained and accordingly sludge recirculation, nutrient addition, draining of excess sludge done. A record shall be maintained for F/M (food to micro-organism ratio), MLSS influent BOD and sludge recycle rate. These parameters should be cross-checked with the design parameters and, if necessary, corrections made to meet the design requirements.
(f) The functioning of biological treatment system is based on microbe activity in polluted water, and hence utmost care is to be taken to keep the system functioning. An efficient biological system helps in treating the wastewater economically.
(v) The cooling tower bleed-off and the boiler blow-down, which are inorganic and have high TDS, may either be solar evaporated or diluted with the other treated wastewater and disposed.
(vi) In case of formulation industries, the levels of wastewater generation are either considerably lower than in the ‘technical’ production or sometimes non-existent. It is observed that most of these industries do not generate any process wastewater.
The remaining plants generate low volumes of highly concentrated wastewater when they wash out reaction vessels or control their emissions using scrubbers. Some industries recycle the vessel washings done using solvents.
The wastewater from these industries may be managed by ‘solar evaporation system’ or contract hauling for incineration thereby maintaining zero discharge.
(vii) The storm-water draining are to be kept separate. The storm water during the first few hours on rainfall, as has highly chances of contaminate with chemical spills etc., should be collected in a tank provided for this purpose having a capacity to handle one hour peak rainfall in a year and treated to the required standards in the ETP before disposal.
The treatment options for wastewater from pesticides industry are given in Fig. 16.24.
Disposal of Treated Wastewater:
The treated wastewater may be reused for floor wash, gardening/irrigation purposes or disposed.
For the usage of treated wastewater for gardening purposes:
(i) The wastewater must be subjected to tertiary treatment by dual-media filtration, chlorination/ozonation, activated carbon filtration etc.
(ii) The dosage rate of wastewater should be limited to 35 kld/hectare/day.
(iii) A storage tank of 2 day wastewater storage capacity should be provided to collect the water and to uniformly use for irrigation/gardening purpose.
(iv) Treated wastewater should be applied on land using sprinklers.
The treated wastewater may be disposed to:
(i) Public sewer.
(ii) Inland surface water.
(iii) Marine coastal water.
For any of these disposals, the treated wastewater should conform to the MINAS.
Designing of Monitoring System:
Monitoring is a systematic collection of data with the objective of:
(i) Assessment of pollution.
(ii) Performance evaluation of pollution control systems.
(iii) Evaluation of impact on the receiving environment.
Carefully designed monitoring network provides the information on resources/material losses in the manufacture of a product identifies pollution sources and problems and helps in arriving at suitable solutions. The role of monitoring as a feedback system for waste management is shown in Fig. 16.25. Monitoring is itself a costly affair.
In order to lay down an optimum monitoring network with respect to cost as well as information effectiveness, the following factors are to be considered:
(i) Minimisation of number of parametres to be monitored.
(ii) Optimisation of frequency of sampling.
(iii) Minimisation of sampling points.
To achieve such monitoring network and to fulfill the objective of assessment of pollution, the following steps are to be considered:
(i) Identification of waste streams.
(ii) Identification of specific pollution parameters considering stoichiometric equations and material balance.
(iii) Evolving a statistical relationship between specific parameters and summary parameters like BOD, COD, and TOC etc.
(iv) Establishing a relationship between production and waste, in terms of specific parameters as wed as summary parameters.
(v) Evolving a quality control chart of summary parameters at each monitoring station with regard to variation within a day and between days.
(vi) Optimisation of the monitoring network by adopting suitable parameters, frequency and monitoring points.
Once the pollution assessment is dene, performance evaluation of effluent treatment plant of various pollution control systems is needed in order to check their performance in achieving the standards prescribed.
The sampling done should be composited hourly for a period, which should not be less than the detention period of a particular treatment unit, viz. neutralisation tank, settling tank, aeration tank, etc.
Otherwise, as the inlet waste characteristics may be varying hourly, the outlet value measured at a particular time by grab sample may not truly represent the amount of treatment done due to greater detention period of the influent in the unit.
The influent theoretically would come out treated after a time period equal to detention time. Hence, sometimes, in grab sampling, the outlet shows higher pollution than the inlet.
So, the frequency of sampling and type of sampling (grab or composite) should be properly selected. In composite sampling, the samples collected each hour are composited in the ratio of wastewater flow to make one final representative sample that is used for testing. Once the performance evaluation of pollution control devices is over, the impact of treated waste on the receiving environment viz. air, water, soil etc. need to be monitored.
In case of disposal to water bodies, the objective is to preserve its designated best use and to have no observed effect on biota due to toxicity. In case of disposal on land, monitoring is needed to know if any changes in soil characteristics are taking place due to the application of effluent.
The following parameters are suggested to be analysed monthly, during the irrigation period and at the beginning and end of the season:
(i) Physical characteristics – Soil texture, permeability, non-capillary erosion, water table depth, topography and water holding capacity.
(ii) Chemical characteristics – Soluble salt content, excess sodium percentage and pH.
(iii) Leachate and surface run-off characteristics – Biochemical oxygen demand, chemical oxygen demand and nitrate.
Also the groundwater should be closely monitored to check the impact due to discharge on land. For this purpose, at least three hand pumps may be provided in the area where wastewater is used for gardening/irrigation purposes.
Effluent Treatment Technology:
A sound organisational set up is needed to look after the environmental management in an industry. The environmental division of an industry should have an environmental specialist to look, into matters related to pollution control and evolve norms for resource conservation/waste minimisation vis-a-vis process control. Besides, he should also evolve norms for optimal utilisation of resources and performance of various pollution control systems.
This division should also have well-trained operators for the treatment facilities, staff to monitor and analyse the wastewater and engineers/technicians to look/ supervise the functioning of the facilities. The ‘technical capabilities’ and ‘attitude’ of the staff are very important for efficient operation of a system and achieving desired results. These staff is to be periodically trained.
To oversee the implementation of the measures for pollution control and the overall management of environment, an Environmental Management Group, comprising members from production, quality control/laboratory, R&D, waste treatment facilities, environmental specialist and top management, should be formed. Pollution control can be cost-effectively achieved only with the involvement of staff from various sectors. Pollution prevented is the profit made and liability reduced.