GB1577036A - Treatment of raw sewage to produce non-toxic solids - Google Patents

Description

(54) TREATMENT OF RAW SEWAGE TO PRODUCE NON-TOXIC SOLIDS (71) We, RICHLAND RESOURCES LIMITED, a British Company of 20, Bridgeman Terrace, Wigan, WN1 1TD, do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to treatment of sewage to produce non-toxic solids which can, for example, be used in fertilisers or animal foodstuffs.

It is usual to treat sewage, which consists mainly of water, so as to clean the water sufficiently for discharge into a river system.

In such treatments, sedimentation and biodegradation steps are carried out to remove solids and to render the solids more suitable for dumping. It has been proposed to clean water by using an electrolytic process to effect flocculation, the flocculated waste material (e.g. detergents, oils, fats, greases and some proteinaceous material) being drawn off and the water then being passed to sedimentation and biodegradation stages for further cleaning. Such a process is described in patent specification no. 1 380 488. It has also been proposed to use the sewage solids to make an animal foodstuff ingredient from sewage by biologically treating the sewage, sterilising and drying. as, for example.

described in patent specification no. 1 275 274. Where there are industrial effluents in the sewage, the resultant food would be contaminated. e.g. by metals, so that this process could only be used with "clean " sewage.

The present invention provides a sewage treatment process for the production of nontoxic solids from raw sewage, the process comprising the steps of mechanical cleaning and degritting of the sewage, settling of the sewage and separating the sediment slurry, acidifying the slurry and adjusting the temperature, if necessary. to at least 5"C to solubilise metal components of the slurry, and dewatering the slurry to remove metal components in the aqueous phase.

The resultant material may be dried, for example, by addition of dry solids, by evaporation, or by pressing or rolling.

A biodegradation step can be used, if required, after removal of metal components.

The pH of the slurry may be adjusted by the acidification step in the range 0 - 5. The acid used is, preferably, hydrochloric acid, but other acids (e.g. sulphuric acid) or mixture of acids may be used to ensure solubility of the particular toxic metals present.

Ligands or chelating agents may be used to capture some metal ions to assist their removal. An oxidising agent may be used to render some toxic substances harmless.

After dewatering to remove most of the metals in solution, the resultant slurry may be subjected to electro-chemical treatment for further removal of metal ions, if the toxicity is particularly high.

After dewatering, the pH may be adjusted to be substantially neutral to pH 5 to 8, if required. This may be effected by addition of calcium compounds, such as lime or chalk.

This step may be carried out before or after sterilisation of the material.

One example of the process according to the invention is now described with reference to the accompanying drawing, wherein the sole figure is a diagram representing steps of the process in block form. Process steps indicated in boxes formed from broken lines are optional or alternative.

The raw sewage including industrial waste is first mechanically cleaned by a coarse screening step 11 for the removal of large bodies, e.g. carcasses and paper using bar screens with 3 inch gaps and then a fine screening step 12 using bar screens with 1/2 inch gaps to remove smaller bodies. After mechanical cleaning, the sewage is subjected to a degritting step 13 by settlement, the liquid flowing along a widening flow path, so that the velocity is reduced to below 1 foot/second. At this stage, more dense particles such as grit and sand settle out leaving a suspension, which includes both organic matter and inorganic matter.

The sewage is then subjected to a primary sedimentation step 14 for 3 to 8 hours, the resultant sediment being drawn off separately from the liquid as a slurry. The slurry is subjected to an acidification step 15 being stirred with 30% hydrochloric acid to adjust the pH to the range 0 to 5 and to a heating step 16, if necessary, to heat the slurry to 5 or above. This solubilises toxic metals such as cadmium, nickel, cobalt and chromium, which are often present in the sewage as suspensions of insoluble solids such as hydroxides and carbonates. At this stage, there may be included the additional step 17 of adding a chelating agent and/or an oxidising agent to assist solubility.

The slurry is then subjected to a dewatering step 18 where water is mechanically removed to provide a sludge of approximately 30% solids. Dewatering may be effected for example, by centrifugation, pressing, rolling or vacuum filtration (with or without flocculation).

Most of the metal ions in solution are drawn off with the liquid, so that a considerable reduction in the concentration of these metals in the dewatered sludge is obtained.

Depending on the original degree of metal contamination, this concentration may be so low as to be acceptable.

Where the concentration of metals is too high, the dewatered sludge may be washed at 30 and subjected to another acidification treatment, perhaps again with the addition of an oxidising agent or chelating agent. This further treatment may be carried out as a secondary treatment or by returning the washed sludge to the main process at 15. The sludge may optionally be subjected to an electrochemical treatment 19 to remove further metal ions. This treatment can be at any desired stage after the acidification step.

The liquid removed at the primary sedimentation step 14 and at the dewatering step 18 may be subjected to metals collection steps 31a and 31b and then passed to a conventional secondary sedimentation step 32.

The sludge may then be neutralised 20a using, for example, chalk or limestone to achieve a pH of 5 to 8. Neutralisation may take place between the chelating or oxidation step 17 and the dewatering step 18.

The sludge is then mixed 21 with dry food material such as straw or brans and any other required additives to make a complete foodstuff component. This mixing step serves to dry the sludge by reducing the overall water concentration.

The foodstuff component is then sterilised 22 in this example by heating in a closed autoclave for 15 to 30 minutes at 1300 to 1600C and under pressure up to 50 p.s.i. This kills parasites and bacteria in the foodstuff component. The foodstuff component is usable and may be bagged 24b in this form.

The foodstuff component may, however, be kiln dried 23 to form a dry sterile powder of 88% solids and bagged 24a in this form and may be submitted to air drying.

A neutralisation step 20b may be carried out after sterilisation instead of at position 20a immediately after dewatering. The acidity may assist sterilisation.

The entire process described above may be continuous.

The liquid removed during the sedimentation step 14 and/or the dewatering step 18 may be treated in conventional manner to render it sufficiently harmless to be discharged into a river system.

Where the secondary treatment 32 produces secondary solids, these may be returned to the main process route at stage 15 and processed with the primary solids.

The resulting product is a richer source of nutrients than the primary solids. However, the secondary solids may be treated separately and can undergo similar treatment to the primary solids commencing at stage 15.

The above described process provides a valuable proteinaceous foodstuff component which may be used in feeding poultry, pigs and cattle. The detoxified solids may instead be used as a valuable source of proteins, fats, oils and other materials which can be more easily extracted once metals are removed, or the material may be used as a fertiliser.

Hitherto, sewage solids have been a waste product whose dumping on land or at sea, or spreading as a fertiliser is causing increasing problems, due to metal contamination.

WHAT WE CLAIM IS: 1. A sewage treatment process for the production of non-toxic solids from raw sewage, the process comprising the steps of mechanical cleaning and degritting of the sewage, settling of the sewage and separating the sediment slurry, acidifying the slurry and adjusting the temperature, if necessary, to at least 5"C to solubilise toxic components of the slurry and dewatering the slurry to remove the toxic components in the aqueous phase.

2. A sewage treatment process according to claim 1, wherein, the pH of the slurry is adjusted during the acidification step in the range 0 - 5.

3. A sewage treatment process according to claim 1 or 2, wherein, after the acidification step, ligands and/or chelating agents are used to capture some metal ions to assist their removal.

4. A sewage treatment process according to claim 1,2 or 3, wherein, after the acidifica

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. liquid flowing along a widening flow path, so that the velocity is reduced to below 1 foot/second. At this stage, more dense particles such as grit and sand settle out leaving a suspension, which includes both organic matter and inorganic matter. The sewage is then subjected to a primary sedimentation step 14 for 3 to 8 hours, the resultant sediment being drawn off separately from the liquid as a slurry. The slurry is subjected to an acidification step 15 being stirred with 30% hydrochloric acid to adjust the pH to the range 0 to 5 and to a heating step 16, if necessary, to heat the slurry to 5 or above. This solubilises toxic metals such as cadmium, nickel, cobalt and chromium, which are often present in the sewage as suspensions of insoluble solids such as hydroxides and carbonates. At this stage, there may be included the additional step 17 of adding a chelating agent and/or an oxidising agent to assist solubility. The slurry is then subjected to a dewatering step 18 where water is mechanically removed to provide a sludge of approximately 30% solids. Dewatering may be effected for example, by centrifugation, pressing, rolling or vacuum filtration (with or without flocculation). Most of the metal ions in solution are drawn off with the liquid, so that a considerable reduction in the concentration of these metals in the dewatered sludge is obtained. Depending on the original degree of metal contamination, this concentration may be so low as to be acceptable. Where the concentration of metals is too high, the dewatered sludge may be washed at 30 and subjected to another acidification treatment, perhaps again with the addition of an oxidising agent or chelating agent. This further treatment may be carried out as a secondary treatment or by returning the washed sludge to the main process at 15. The sludge may optionally be subjected to an electrochemical treatment 19 to remove further metal ions. This treatment can be at any desired stage after the acidification step. The liquid removed at the primary sedimentation step 14 and at the dewatering step 18 may be subjected to metals collection steps 31a and 31b and then passed to a conventional secondary sedimentation step 32. The sludge may then be neutralised 20a using, for example, chalk or limestone to achieve a pH of 5 to 8. Neutralisation may take place between the chelating or oxidation step 17 and the dewatering step 18. The sludge is then mixed 21 with dry food material such as straw or brans and any other required additives to make a complete foodstuff component. This mixing step serves to dry the sludge by reducing the overall water concentration. The foodstuff component is then sterilised 22 in this example by heating in a closed autoclave for 15 to 30 minutes at 1300 to 1600C and under pressure up to 50 p.s.i. This kills parasites and bacteria in the foodstuff component. The foodstuff component is usable and may be bagged 24b in this form. The foodstuff component may, however, be kiln dried 23 to form a dry sterile powder of 88% solids and bagged 24a in this form and may be submitted to air drying. A neutralisation step 20b may be carried out after sterilisation instead of at position 20a immediately after dewatering. The acidity may assist sterilisation. The entire process described above may be continuous. The liquid removed during the sedimentation step 14 and/or the dewatering step 18 may be treated in conventional manner to render it sufficiently harmless to be discharged into a river system. Where the secondary treatment 32 produces secondary solids, these may be returned to the main process route at stage 15 and processed with the primary solids. The resulting product is a richer source of nutrients than the primary solids. However, the secondary solids may be treated separately and can undergo similar treatment to the primary solids commencing at stage 15. The above described process provides a valuable proteinaceous foodstuff component which may be used in feeding poultry, pigs and cattle. The detoxified solids may instead be used as a valuable source of proteins, fats, oils and other materials which can be more easily extracted once metals are removed, or the material may be used as a fertiliser. Hitherto, sewage solids have been a waste product whose dumping on land or at sea, or spreading as a fertiliser is causing increasing problems, due to metal contamination. WHAT WE CLAIM IS:

1. A sewage treatment process for the production of non-toxic solids from raw sewage, the process comprising the steps of mechanical cleaning and degritting of the sewage, settling of the sewage and separating the sediment slurry, acidifying the slurry and adjusting the temperature, if necessary, to at least 5"C to solubilise toxic components of the slurry and dewatering the slurry to remove the toxic components in the aqueous phase.

2. A sewage treatment process according to claim 1, wherein, the pH of the slurry is adjusted during the acidification step in the range 0 - 5.

3. A sewage treatment process according to claim 1 or 2, wherein, after the acidification step, ligands and/or chelating agents are used to capture some metal ions to assist their removal.

4. A sewage treatment process according to claim 1,2 or 3, wherein, after the acidifica

tion step, an oxidising step is carried out.

5. A sewage treatment process according to any preceding claim, wherein the water removed at the dewatering step is subjected to a metals removal process and sedimentation.

6. A sewage treatment process according to any one of claims 1 -5, wherein the sludge obtained from the dewatering step is washed and resubjected to acidification.

7. A sewage treatment process according to any preceding claim wherein the sludge obtained from the dewatering step is additionally subjected to electrolytic treatment to remove metals.

8. A sewage treatment process according to any preceding claim, wherein the detoxicated sludge is dried and sterilised.

9. A sewage treatment process according to claim 8, wherein the sludge is mixed with other materials to form an animal foodstuff ingredient.

10. A sewage treatment process substantially as herein described with reference to the accompanying drawing.