GB2225317A - Process for treatment of sewage within sewers - Google Patents

Abstract

A method of sewage treatment in the sewage conveyance system which involves the addition of nitrate salt alone or in combination with an alkali and a bacterial nutrient to sewage flowing in a sewer or at a pumping station so as to allow aerobic digestion of the sewage to proceed and also to retard or prevent anaerobic fermentation of the sewage within the sewerage system.

Description

PROCESS FOR TREATMENT OF MIXED ORGANIC AND INORGANIC BIODEGRADABLE WASTES WITHIN SEWERAGE CONVEYANCE SYSTEMS This invention relates to a process for Aerobic biochemical treatment and stabilisation of waterborne organic and inorganic waste matter, particularly but not exclusively for treatment of sewage being conveyed within sewerage systems or other confined spaces.

At present sewers and drains are used exclusively for the conveyance of sewage and other waterborne waste matter to a suitable disposal or treatment location, usually controlled by a Government Authority. Sewers and drains are not normally considered as treatment units for sewage but only as waste water conveyance systems.

Sewerage collection and conveyance systems generally comprise a network of sewers and drains leading to a series of sewage pumping stations or a single sewage pumping station or by direct drainage via a main sewer leading to a point of discharge at a sewage treatment plant or other suitable discharge point (River, Estuary or Sea).

Problems arise within this network due to biological degradation of untreated sewage, particularly when Anaerobic fermentation of waste material occurs. Anaerobic fermentation of sewage is commonplace when raw sewage is handled or conveyed in confined spaces, particularly, but not always so during hot climatic conditions.

The metabolic products and by-products of Anaerobic fermentation of sewage may include explosive gases such as methane and carbon monoxide, odorous toxic gases such as hydrogen sulphide and mercaptans, corrosive substances such as organic acids, also sulphuric acid may be formed by the bacteriological reaction between hydrogen sulphide gas and any available oxygen present in the sewer atmosphere.

Anaerobic metabolic gaseous products are not only a health hazard but may lead to serious degradation of sewer fabrication materials and mechanical and electrical equipment installed at pumping stations and other parts of the sewage handling system.

The odorous gases may also be a serious nuisance to the general public where these are allowed to escape from the sewerage conveyance system.

There have been many attempts to overcome the problems due to Anaerobic fermentation within the sewerage network. Various designs of sewers and construction materials therefore have been used. also chemical precipitation or neutralisation of specific troublesome products have been used. Toxic and oderous gas extraction systems have been installed at some locations within sewerage systems with only limited success as these do not deal with the source of the problem.Pure oxygen generation and injection systems have been employed in an effort to maintain Aerobic conditions in sewage pumping MAINS, However this practice has a very limited application as it cannot safely be used in GRAVITY sewers owing to the risk of explosion, also its use in steel pumping mains with air release valve chambers can be very troublesome, dangerous and corrosive, also such technologically complex systems and methods are expensive and difficult to operate in less highly developed countries.

Domestic sewage and other waterborne wastes can be treated and stabilised by admixtures of Aerobic saprophytic bacteria and micro-organisms in the presence of oxygen dissolved from the atmosphere, or oxygen generated artificially. This is the basis of most sewage# treatment processes currently being used, and the efficacy of such systems are dependant on both the concentration of the saprophytic biomass and of the dissolved oxygen available to satisfy the respiration rate of the biomass.

It has been calculated and demonstrated by the Inventor that within sewerage systems there is normally a sufficient concentration of active saprophytic biomass available, in the form of bacteriological slimes and deposits, for the sewage entering the system to be successfully treated. However-this is seldom possible due to the lack of sufficient atmospheric dissolved oxygen within an enclosed sewerage system to meet the high respiration rate of the Aerobic saprophytes and thus allow treatment of the sewage to proceed satisfactorily.

According to the present invention a method of Aerobic biochemical treatment of raw sewage within a sewerage conveyance system and the prevention of harmful Anaerobic products as mentioned herein comprises the step of addition of a soluble nitrate salt alone or in combination with a suitable alkali or other bacterial nutrient to sewage flowing in a sewer or at a pumping station, the mixture being added in a proportion sufficient to allow Aerobic digestion of the sewage to proceed also to retard or prevent Anaerobic fermentation of the sewage within the sewerage system.

The proportion is preferably selected and injected to the sewerage system at a suitable location to avoid any deleterious effect on the final treatment processes at the sewage treatment works and to ensure that no nitrate remains in the. sewage at the final point of discharge from the sewerage network. The nitrate serves as a source of metabolic oxygen for the bacteria and micro-organisms in the sewage and the bacterial slime growths on the sewer walls and floors, whilst the alkali, added where necessary, raises the pH value of the sewage to prevent the liberation of Hydrogen Sulphide and to re-dissolve any previously formed hydrogen sulphide gas; the latter being the main source of sewer problems. Other bacterial nutrients may be added where considered necessary to enhance the Aerobic bacterial metabolism, thus increasing the biological treatment capacity of the Sewerage system.

The nitrate (or mixture) is preferably added as a measured solution to the sewerage system upstream of a pumping station, or other suitable and convenient injection point(s), and preferably at or near the source of the sewerage network.

Further amounts may be added downstream of the point(s) of the initial application location(s). Generally the solution will be added in a proportion related to the flow: of sewage within the sewerage network and in concentrations sufficient to maintain Aerobic digestion of the sewage wastes.

Use of calcium nitrate is preferred although other economically viable nitrates may be employed. The addition of alkalis in admixture with the nitrate will depend on the inherent pH value of the sewage to be treated within the system; example in soft water areas or at locations where desalinated water is used for domestic potablewater supplies then the addition of alkali will be more important than in hard water areas where the pH value will be higher.

The proportion of nitrate added will depend on the biochemical oxygen demand of the sewage being conveyed and treated, also of the bacterial biomass respiration activity within the sewerage network. This depends on such factors as the nature, strength, temperature and other components of the sewage being conveyed, also the length of time that the sewage remains in the sewerage network and the concentration of active bacterial biomass within the sewerage system (Slimes, silts and suspensions).

The proportion of nitrate added will also depend on the degree of Biochemical oxidation treatment required by the User. Example, the User may only require partial treatment of the sewage inorder to reduce the biological loading on existing treatment units, thus avoiding the high capital costs of works extensions.

The use of nutrients to enhance bacterial activity and other chemicals such as ferric salts to precipitate any sulphides already present in the sewage may be added. Also hydrogen peroxide may be used with the nitrate mixture to provide an initial source of easily available oxygen to promote rapid Aerobic metabolism; bacterial reduction of nitrate being a more gradual and comparatively slower process The product gases produced by the reduction of nitrate in Aerobic decomposition of sewage are nitrogen and carbon dioxide which are both non toxic or explosive gases and should not therefore produce a serious hazard within the sewerage network.

This invention is NOVEL in so far as it seeks to utilise the sewerage conveyance network as a system of partial purification of sewage by the use of certain chemicals under controlled conditions whilst ensuring safety within the system as a whole, reducing the strength of sewage to be treated at the sewage works and to reduce or eliminate the existing serious hazards and odour nuisance associated with decomposing sewage under Anaerobic conditions. Also and of equal value, seeks to considerably reduce the cost of sewer maintenance and treatment of sewage at purification works. An added advantage is that the sewage being conveyed within the sewerage network will be less hazardous to maintenance operators, also on arrival for further processing at the sewage treatment works it will be less noxious to handle and cause less problems for operators and nearby residents.

Experimental Evidence Experimental proof that the foregoing Invention is a viable and practical method of successfully treating sewage within a sewerage system has been undertaken by the Inventor on both laboratory and full scale tests at a location where the hazards and problems related in the Patent invention were considerable.

Laboratory Experiments These were carried out on samples of raw sewage and sewage slimes taken from a domestic sewerage system and were carried out to determine the percentage reduction in sewage pollution relating to the concentration of chemical nitrate required.

Also to determine the reduction and elimination of hazardous products by the use of chemical nitrate.

Claims (6)

1. A method of treatment of Sewage comprising the utilisation of at least part of a sewerage conveyance network in at least partial purification of sewage.

2. A method of treatment of sewage as claimed in claim 1, wherein said method comprises the addition of a nitrate salt to said sewage in the sewerage network to cause aerobic digestion of at least a part of the sewage.

3. A method of treatment of sewage as claimed in claim 2 wherein said nitrate salt comprises calcium nitrate.

4. A method of treatment of sewage as claimed in claim 2 and 3 wherein the quantity of nitrate salt added to said sewage is such as to substantially retard anaerobic fermentation of said sewage.

5. A method of treatment of sewage as claimed in any preceding claim, wherein said method further compromises the addition of an alkali to said sewage in said sewerage network.

6. A method of treatment of sewage as substantially hereinbefore defined with reference to and tabulated in the results of trials.