baffle walls in the centre of the channel at an angle to the direction of flow. Advantages of Activated Sludge Process. The advantages of activated sludge treatment are as follows: It requires little space, causes remarkably little smell, and it is a very flexible form of treatment, for the time of aeration, quantity of air and proportion of sludge can all be varied to give the desired result. Its disadvantages are that working costs are higher than those of sedimentation and filtration works, that skilled management must be in attendance, that it is very sensitive to changes in the character of the sewage and that enormous quantities of sludge are produced. It may also be mentioned that occasionally there is a sudden increase in the sludge in the aeration tank to more than twice its normal volume, and that this is accompanied by a deterioration of the effluent. This is known as “bulking” and is due to presence of protozoa and other growths, brought about apparently by insufficiency of air. It can be remedied by prolonged aeration. A most noteworthy fact, in the activated sludge process, is the rapidity with which organic matters are oxidised when the sewage is first brought into contact with the sludge and the slowness with which it is improved by more prolonged treatment. For example, at Birmingham it was found that 60 per cent. of the impurities were oxidised in one hour and that only a further 32 per cent. were oxidised in the next five hours. It seems therefore that, although it is possible to effect satisfactory purification by sufficiently prolonged activated sludge treatment and settlement alone, it is more economical to aerate for about one hour only and to complete the oxidation of organic matters in percolating filters. After the one hour’s aeration and subsequent settlement of sludge, the sewage will be quite free from colloidal matters and well charged with oxygen, and can be passed through the percolating filters at twice the ordinary rate. When this arrangement is adopted it is, however, necessary to re-activate the settled sludge, before returning it to the sewage entering the aeration tanks, since the sludge has more work to do, as it were, when it is brought into contact with fresh sewage every hour, than it would be required to perform if it worked on the same sewage for six or eight hours, which would be about the time required for full purification. The activated sludge is not well adapted to small plants.<Callout type="important" title="Important">Activated sludge treatment requires skilled management and cannot be easily implemented in small-scale systems.</Callout> Disposal of Sludge. One of the greatest troubles in sewage disposal works of any size is the disposal of sludge. Formerly it was sometimes possible to dispose of it in a crude state by sale as manure, but this is now practically impossible and the problem is to get rid of it at the least possible cost. The principal methods are as follows: 1. Barging it out to sea—a cheap process for places conveniently situated for so doing. This is done in the case of London, Manchester, Southampton and other places.<Callout type="risk" title="Risk">Barging sludge can be expensive if not located near a suitable harbor.</Callout> 2. Covering flat land with a layer of a few inches of the wet sludge. This rapidly dries, but the system is unsuitable for strongly smelling sludge.<Callout type="warning" title="Warning">Avoid using this method in areas where strong odors are problematic.</Callout> ’ 3. Shallow burial in the ground. Wet sludge is run into trenches about 24 to 80 inches wide and 12 to 18 inches deep, spaced about 3 feet apart. When the sludge has dried to a firm state it is covered with a thin layer of soil. After a period of not less than one month the land is ploughed up and planted with crops.<Callout type="tip" title="Tip">This method can be used for agricultural purposes, reducing disposal costs.</Callout> 4. Lagooning or air drying. In this method earth tanks, from 2 to 4 feet deep, are made by excavation and by forming banks with the excavated material, and under-drained with 8 inch or 4 inch diameter agricultural land drains, spaced at intervals of about 9 feet. The bottom of the tank is then covered with a layer of 6 inches or more of clinker or ashes. Sludge is then run in, or pumped in, and allowed to remain there for some months. When moisture has drained and evaporated sufficiently for digging out, it will be found to have shrunk to about one-half its original volume.<Callout type="important" title="Important">Lagooning can be a cost-effective method but may cause nuisance from smell.</Callout> 5. Pressing it into cakes in special presses. This process reduces moisture content to about 50 per cent. and the bulk to about one-fifth. If the district is agricultural the cakes may be saleable; in other cases they are used for filling low land, dug into land, barged to sea or mixed with house refuse and burnt in refuse destructors.<Callout type="risk" title="Risk">Pressing sludge can be expensive due to specialized equipment requirements.</Callout> 6. By digestion in tanks. This is a process which has been developed greatly in recent years at Birmingham and elsewhere. It is a septic tank treatment applied to sludge after its separation from sewage, the sludge being usually inoculated by an admixture of already digested sludge.<Callout type="important" title="Important">Sludge digestion can be used as fertilizer but requires careful management.</Callout> Disposal Schemes for Country Houses without Main Drainage. One or two examples of small installations may now be given for the disposal of the sewage of country houses, or small institutions which are not connected to public sewers.<Callout type="gear" title="Gear">Chemical precipitation systems and septic tank installations require specific equipment.</Callout> Chlorine Treatment to Prevent Odour and Stop “Ponding”’ of Filter Beds. If the disposal plant has to be rather near the house and on the side of the prevailing winds, there may be some noticeable odour at the house or in the grounds nearby and if the plant cannot be placed farther away and on the side where prevailing breezes will carry such odours away rather than to the house, it may be worth while to install an automatic chlorination equipment. An expert in chlorination should be consulted, for over-dosing would kill off the bacteria and stop the bacterial action, but correct dosing (either in liquid or gaseous form or as sodium hypochlorite and bleaching powder) reduces odour without interfering with the bacterial action.<Callout type="warning" title="Warning">Over-chlorination can harm beneficial bacteria.</Callout> It also helps to keep the filtering material in a clean condition and prevents the film of green slime (due to algae) on the surface of the filter beds, holding up the effluent in large puddles (or ponds), and allowing the effluent to run too fast through the filter bed in the remaining parts which are not so affected. Public sewage-disposal plants sometimes cure this tendency of the filter beds to “pond up” by inoculating the surface with a few pieces of clinker from a nearby sewage works where the beds are known to be clear.<Callout type="tip" title="Tip">Inoculation can help maintain clean and efficient filter beds.</Callout> This inoculation introduces a few specimens of a small grey-blue insect, known as a “springtail” (Acherutes Viaticus). These soon multiply and feed upon the algae, thus keeping the surface clean. In small bacteria beds for private houses, it is generally enough to rake over the surface occasionally.<Callout type="important" title="Important">Regular maintenance can prevent filter bed clogging.</Callout> Septic Tank Installations. In Figs. 429-431 a septic tank installation is shown, the system involving the use of a covered septic tank and successive filtration. Such a system is applicable to the case of a house on a hill-side, or any position in which a good tall is obtainable.<Callout type="gear" title="Gear">Covered septic tanks require careful design for effective operation.</Callout> The sewage enters at A, passing through the inlet chamber, I.C., and the grit chamber, G.1C.1, from which it enters the septic tank. The tank, as shown in Fig. 480, is covered and ventilated, and has two manholes for access. Both inlet and outlet are submerged, and the sewage passes into the ponding chamber, P.C., from which it can circulate through the circulating chamber, C.C., as shown in Fig. 431. It will be noted that there are non-return valves, V.V., between the ponding and circulating chambers.<Callout type="important" title="Important">Non-return valves prevent backflow and ensure proper sewage flow.</Callout> From the ponding chamber the tank liquid passes into the automatic gear in the chamber X., and thence into two distributing pipes over the top of the high-level filter. The effluent from this filter is collected in drains on its floor, leading to the discharge well, D.W., adjoining the ponding chamber, and from there it is permitted to pass to the low-level filter, from which the final effluent is discharged into the nearest water-course.<Callout type="gear" title="Gear">Automatic gear ensures efficient distribution of sewage.</Callout> The sludge from the septic tank is collected through a special form of perforated pipe, and led to a cleansing shaft, C.S., the floor of the circulating chamber connecting with the same pipe. From the cleansing shaft the sludge can be led in any desired direction.<Callout type="important" title="Important">Proper sludge management prevents clogging.</Callout> Smaller Installation for House on Flat Site. Figs. 482 to 484 show an example in which the site is rather flat, and the open tank can be put at such a distance from the house and road as not to permit of nuisance from smell.<Callout type="gear" title="Gear">Proper placement of tanks reduces odor issues.</Callout> The sewage enters a grit chamber, G.1C.1, in which is a screen. From the grit chamber an overflow or emergency drain is shown in dotted lines, the entrance to it being controlled by a small sluice, S.<Callout type="important" title="Important">Grit chambers help prevent damage from large particles.</Callout> The sewage passes from the grit chamber, past a sluice S, to the septic tank, S.T., the inlet being submerged. The floor of this falls, leading past a valve, V., to the sludge chamber, S.C., from which the sludge can be removed by pumping or other means.<Callout type="important" title="Important">Regular sludge removal is crucial for system efficiency.</Callout> The outlet for the liquid from the septic tank is submerged, and leads to a distributing chamber, D.C., a very shallow chamber, as shown in Fig. 433. Adjoining the distributing chamber are two filters, F., of circular plan.<Callout type="important" title="Important">Filters ensure clean effluent before discharge.</Callout> In the centre of each is a collecting or dosing tank, D.T., which actuates a rotary sprinkler, R.S. The dosing tank is fed from the distributing chamber by a feed pipe, F.P., and the chamber can be flushed out by means of a valve, V., leading direct to the filter.<Callout type="important" title="Important">Rotary sprinklers ensure even distribution.</Callout> The filtered effluent passes from the bottom of the filters into the effluent drain, E.D., a manhole being placed near the filters for the purpose of sampling the effluent from them. The three examples just given will serve the purpose of showing the methods of treating varied conditions from varied points of view.<Callout type="important" title="Important">These systems can be adapted to different site conditions.</Callout> When it is thought desirable to use automatic alternating gear in or alongside the dosing chamber, this can be done very simply by arranging a copper float which rises and falls with the effluent in the dosing chamber, at the same time turning bevel gears, which can be devised so as to transmit the horizontal twisting motion of the horizontal axle to a corresponding motion in the vertical rod in the alternating gear chamber shown in Fig. 435. The bottom of this rod can then carry a circular dise (contrived with a ratchet and the gearing) to alternately open and close the entrance to pipes A and B, leading right and left, to the contact beds or the percolating filters shown in Figs. 427 and 428, or similar installations.<Callout type="important" title="Important">Automated systems can optimize sewage treatment.</Callout> It should be pointed out that the architect of country houses and institutions requiring sewage-disposal plant, having decided the type of disposal scheme most suitable to the conditions, usually leaves the design of the details in the hands of a specialist firm, of whom a number are available in London and most large towns.<Callout type="important" title="Important">Specialist firms can provide tailored solutions for complex systems.</Callout>
Key Takeaways
- Activated sludge process is flexible but requires skilled management and cannot be easily implemented in small-scale systems.
- Proper sludge management, including regular removal and inoculation with beneficial organisms, is crucial for system efficiency.
- Various methods of sludge disposal exist, each with its own advantages and drawbacks.
Practical Tips
- Implement a septic tank installation if you are not connected to public sewers, ensuring proper sewage treatment and management.
- Regularly check and maintain your filter beds to prevent clogging from algae growth or other debris.
- Use automatic chlorination equipment to reduce odors and keep filter beds clean.
Warnings & Risks
- Over-chlorination can harm beneficial bacteria in the septic tank, leading to reduced treatment efficiency.
- Incorrect disposal of sludge can cause environmental pollution and health hazards.
- Inadequate management of activated sludge process can lead to increased costs and inefficiencies.
Modern Application
While many historical sanitation techniques have been improved upon with modern technology, the principles of efficient sewage treatment and proper sludge management remain crucial. Understanding these methods can help in preparing for emergencies where public infrastructure may be compromised or unavailable.
Frequently Asked Questions
Q: What are the main advantages of the activated sludge process?
The main advantages include requiring little space, causing remarkably little smell, and being a very flexible form of treatment. The time of aeration, quantity of air, and proportion of sludge can all be varied to give the desired result.
Q: What are some methods for disposing of sewage sludge?
Methods include barging it out to sea, covering flat land with wet sludge, shallow burial in the ground, lagooning or air drying, pressing into cakes using special presses, and digestion in tanks. Each method has its own advantages and drawbacks.
Q: How can chlorine treatment help prevent odors in sewage disposal systems?
Chlorine treatment reduces odour without interfering with the bacterial action by killing off harmful bacteria that cause unpleasant smells. It also helps to keep the filtering material clean and prevents green slime from forming on filter beds.