Waste water Treatment, Chemistry tutorial

Introduction

In modern societies, proper management of wastewater is a necessity, not an alternative. The practice of collecting and treating wastewater prior to disposal is comparatively a recent undertaking. It began to increase the attention of the stakeholders only when it became apparent that the 'available water' is inadequate to support the available people and their needs. Earlier, city dwellers (of London, Paris and Boston, for instance) used to place "night soil" in buckets along the streets and workers would empty the waste into "honey wagon" tanks. The waste was transported to rural areas for disposal over agricultural land.

The invention of the flush toilet in the 19th Century radically transformed waste-disposal practices. Existing systems for transporting urban wastes for disposal on agricultural earths were not sufficient to grip the huge volume of liquid produced through the flush toilets. Faced via this transportation difficulty, cities began to utilize natural drainage systems and storm sewers for wastewater carriage. Because storm drain systems obviously ended at watercourses, the allegation of this practice was that waterborne wastes were released straight to streams, lakes and estuaries with no treatment these that the self-purification capacity of the receiving streams was exceeded and nuisance conditions became intolerable. Gross pollution often resulted and health problems were conveyed from the sewered community to downstream users of water.

The first "modern" sewerage system for wastewater carriage was built in Hamburg, Germany in the year 1842 via Lindley. Lindley's system involved many of the principles that are still in use today. Advanced wastewater- treatment processes have been developed and are still being perfected to produce potable water from domestic wastewater.

Wastewater characteristics

Wastewaters are usually classified as industrial wastewater or municipal wastewater. Industrial wastewater by traits compatible through municipal wastewater is often discharged to the municipal sewers. Many industrial wastewaters require pretreatment to remove non-compatible substances prior to discharge into the municipal system.

Municipal wastewater,  having  been  put  to  a  wide  variety  of  uses, contains  a wide variety of contaminants: suspended solids, biodegradable organics, pathogens, nutrients, refractory organics, heavy metals and dissolved inorganic  solids. Suspended solids are primarily organic in nature and are composed of some of the more objectionable materials in sewage. Body wastes, food wastes, paper, rags and biological cells from the bulk of suspended solids in wastewater. Suspended solids cause sludge deposits and anaerobic conditions in aquatic environments. Their removal is essential prior to discharge or reuse of wastewater.

Although suspended organic solids are biodegradable through hydrolysis, biodegradable   materials in wastewater are   usually considered to be soluble organics such as proteins (40 - 60percent), carbohydrates (25-50percent) and lipids (~ 10percent). All of these materials contain carbon that can be converted to carbon dioxide biologically thus, exerting an oxygen demand.  Proteins also contain nitrogen; thus a nitrogenous oxygen demand is also exerted. The biological oxygen demand (BOD) is used to quantify biodegradable organics.

All forms of waterborne pathogens - bacteria, viruses, protozoa and helminths may be found in domestic wastewater. These organisms are discharged  by  persons  who  are  infected  with  the  disease  and  they transmit communicable diseases. Although pathogens causing some of the more exotic diseases may rarely be present, it is a safe assumption that a sufficient number of pathogens are present in all untreated wastewater to represent a substantial health hazard. Fortunately, few of the pathogens survive wastewater treatment in a viable state.

Traditional wastewater-treatment processes are designed to reduce suspended solids, biodegradable organics and pathogens to acceptable levels before disposal. Additional treatment is required to reduce levels of nutrients if the wastewater is to be discharged to a delicate ecosystem. Processes to remove refractory organics and heavy metals and to reduce the levels of dissolved inorganic solids are required where wastewater reuse is anticipated.

Terminologies in Wastewater Treatment

Unit Operations/Unit Procedures: These are techniques utilized for treating municipal wastewaters. Unit operations engage contaminant elimination via physical forces while unit processes engage biological and/or chemical reactions. Even though unit operations and processes are natural phenomena, they might be initiated, enhanced or controlled via altering the environment in the reactor.

Reactor: This is the vessel or containment structure, along through all of its appurtenances, in that the unit operation or procedure occurs.

Wastewater-treatment System: This is composed of a amalgamation of unit operations and unit processes designed to diminish indeed constituents of wastewater to an suitable level.

Primary Treatment:  The purpose is to eliminate solid substances (huge debris, inorganic solids and organic suspended solids) from the incoming wastewater.

Secondary Treatment: Generally, this consists of biological conversion of dissolved and colloidal organics into biomass that can subsequently be eliminated via sedimentation.

Tertiary Treatment: This frequently involves additional removal of suspended solids and/or the elimination of nutrients from secondary treatment wastewater.

Conventional wastewater treatment

Conventional treatment is the term utilized to explain the standard technique of sewage   treatment. It comprises 4 stages of treatment: (i) preliminary treatment, (ii) primary or physical treatment (sedimentation) (iii) secondary or biological treatment (biofilteration or activated sludge), and (iv) sludge treatment (anaerobic digestion of the sludges).

Preliminary Wastewater Treatment

This is the 1st phase of sewage treatment. It deals through the elimination of huge suspended or floating objects (rags, maize cobs, pieces of wood) and weighty mineral elements (sand and grit). This is essential to stop the equipment utilized in the subsequent phases of treatment from being injured. Preliminary treatment comprises screening, grit removal and comminutor (a common alternative to screening).

Screening: Course solids are eliminated through a series of directly spaced mild steel bars situated across the flow generally inclined at 60o. There are hand-raked screens and mechanically raked screens. Screenings are mainly obnoxious together in appearance and content, and should be buried or incinerated or macerated depending on the volume of the screenings.

Grit removal: Grit (detritus) is the heavy inorganic fraction of  sewage solids  such  as  road  grit,  sand,  eggshells,  ashes,  charcoal, glass  and pieces of metal. It may also contain some heavy organic matter such as seeds, coffee grounds, yam and plantain peels and so on and so forth. The grit is either used for land fill or disposed of by burial.

Comminution: A comminutor is a self-cleansing shredding machine which cuts up sewage solids as they pass or are pulled through a fine screen which forms the outer periphery of the machine. Comminutors avoid the problems associated with handling and disposal of screenings, and for this reason, they are popular with plant operators. Sedimentation:  Sedimentation is the primary or physical stage of wastewater treatment involving the gravitational separation of suspension into its component solid and liquid phases. The aim is to produce high degrees of both clarification and thickening. Clarification is the removal of solids from the liquid phase and thickening the removal of liquid from the solid or sludge phase. A high degree of clarification is required to reduce the load on the secondary (biological) treatment plant and a high degree of thickening is desirable so that sludge handling and treatment (which usually accounts for 30percent of the total cost of conventional treatment) is minimized.

Biofilteration: Biofilteration (secondary or biological treatment) is a stage in which the liquid effluent (settled sewage) from primary sedimentation tanks is treated in one of two biological reactors - a biofilter or an activated sludge process. The biofilter (also known as the percolating, trickling or biological filter or bacteria bed) is a circular or rectangular bed of coarse aggregate of about 1.8m deep. Settled sewage is distributed over the bed and trickles down over the surface of the aggregate. On these surfaces, a microbial film develops and the bacteria, which constitute most of this film, oxidise the sewage as it flows past. As the sewage is oxidised, the microbial film grows.

Some of the new cells so formed are washed away from the film by the hydraulic action of the sewage. These cells exert a high BOD and must be removed before the effluent is finally discharged. This is achieved in secondary sedimentation tanks (humus tanks).  Secondary tanks are basically similar to primary tanks, but without scum-skimming facilities. The clarified effluent is discharged usually to a river and the humus sludge pumped to the sludge pretreatment unit.  

Activated Sludge. Activated sludge is the conventional alternative to biofilteration. Settled sewage is led to an aeration tank where oxygen is supplied either by mechanical agitation or by diffused aeration. The bacteria which grow on the settled sewage are removed in a high-rate secondary sedimentation tank. The sludge contains some inert solids, but the major components making up its loose, flocculent structure are living or active bacteria and protozoa, therefore the name 'activated sludge'.

Sludge Treatment: Primary and secondary sludges are most commonly treated together in a two-stage anaerobic digester.  The first stage is heated, if necessary, to 30 - 33oC so that digester can proceed more quickly; the methane gas released in the digestion process is commonly used to heat the digester contents. An alternative operating temperature is 50oC which permits rapid digestion of the sludge by thermophilic bacteria. The second stage is a thickener for quiescent solids separation. The supernatant liquor has a BOD5 of 5000 - 10000 mg/L and is returned to the main works inlet for complete treatment. The digested sludge, in hot climates, is most advantageously placed on drying beds.

When dry, it may be sold as fertilizer because the nutrient value of the sludge is beneficial to vegetation and its granular nature might serve as a soil conditioner. Tertiary wastewater treatment involves the removal of nutrients and additional components so as to reinstate wastewater to potable quality. It engages nitrogen, phosphorus and solids elimination.

Hot climate challenges

Operation and Maintenance: Conventional sewage treatment relies greatly on compound electrical machinery that needs considerable skills in installation, operation and maintenance. This skill, particularly in maintenance, isn't readily available in many of the humid expanding countries including Nigeria. Therefore, whenever any of the installation isn't working satisfactorily or has stopped working, the usual experience is to abandon the complete plant.

Odour: In hot climates, sewage can soon become septic (malodorous) if sufficient oxygen isn't made obtainable to stop the onset of anaerobic situations. A higher level of odour can therefore be supposed in hot climates to come from primary sedimentation tanks that are, via their nature, intended for quiescent settling and not turbulent oxygenation.

Insect nuisance: The microbial film in biofilters is utilized as a breeding ground via dissimilar flies and midges. In a sense, this is beneficial in that the larvae feed on the film and therefore help to prevent ponding. However, although none of the species found in filters actually bites humans, their sheer numbers can be a severe nuisance in hot climates. For instance, clouds of Psychoda flies can efficiently stop all human activities in and near a sewage treatment works.

Wastewater reuse

Reuse of totally treated wastewater (referred to as reclamation) might be dictated via several circumstances; this whole treatment of wastewater can seldom be defensible except in water-scarce areas where several form of reutilize is mandatory. In such places, waste water might constitute a main portion of the available resource. Where fragile ecosystems necessitate stringent effluent requirements, reuse of the wastewater might assist to offset the cost of progressed wastewater treatment, or a reuse that will agree to a lower level of treatment might obviate the require for the cost of tertiary treatment prior to discharge.

Several of the purposes for that wastewater has been reutilized comprise (i) recreational facilities (for instance swimming and skiing), (ii) industrial water provide (for instance for cooling processes), (iii) groundwater recharge (for instance to stabilize groundwater table), (iv) potable water systems (for instance in pipe-to- pipe recycling or closed loop).

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