By
Abaraogu, Udechukwu John and Dr. Ukpong, E. C.
Civil Engineering Department
Faculty of Engineering University of Uyo, Nigeria.
ABSTRACT
In this work, the physical chemical and biological water treatment of raw and filter water through sand filtration in Kwa community of Cross River State, Nigeria were compared to the Nigeria and world Health organization standards. The mean results after the experiment showed that PH value for the filtered water is 6.6mg/l from 7.6mg/l when it was unfiltered, suspended solids droped from 12.29 (for raw water) to 7.7mg/l after filtration colour reduced from 12.22 TCU to 10143TUC after filtration Turbility was from 14.4NTU to 8.2NTU, dissolved oxygen from 4.0mg/l to 3.5mg/l ammonia from 0.32mg/l to 0.02mg/l, sodium from 30.5mg/l to 25.5mg/l, chloride from 12.1mg/l to 17.1mg/l, total dissolved solids (T.D.S) from 11.4mg/l to 25.76mg/l, the conductivity was from 18.06MS/Cm t0 42.16MS/Cm and the Total coliform count was from 0.043CFU/ML to 0.0217CFu/ML. These figures showed that the sand filtration was very good in treating the water to correspond with the Drinking water standard.
1.0 INTRODUCTION
Filtration is the process of passing water through material to remove particle and other impurities, including floc, from the water being treated. These impurities consist of suspended particles (fine silts and clays), biological matter (bacteria, Plankton, spores cysts or other matter) and floc. The materials used in filters for public water supply is normally a bed of sand, coal or other granular substance.
Filtration does not remove dissolved solids, but may be used together with a softening process which does reduce the concentration of dissolved solids. The sample mechanisms involved in infiltration are: physical and chemical adsorption, straining, sedimentation, interception, diffusion and inertial compaction.
There are different types of media materials used in filtration processes which includes; Quatz sand, silica sand, anthracite, coal, garnet, magnetite. The size and shape of the filter media affects the efficiency of the solids removal from the water being treated. Sharp angular media form large voids and remove less time materials than rounded media of equivalent size. The media must be coarse enough to allow solids to penetrate the bed for 50.8 to 101.6mm. Although most suspended solids are trapped it the surface or in the first 25.4 to 50.8mm of bed depth, some penetration is essentially to prevent a rapid increase in pressure drop.
TABLE 1: EFFECTIVE SIZES AND SPECIFIC WEIGHTS OF DIFFERENT MEDIA
| S/n | Media | Effective size (mm, in) | Specific gravity |
| 1 | Anthracite | 0.3-1 | 1.4 |
| 2 | Garnet | 0.4-0.6 | 3.8 |
| 3 | Magnetite | 0.3-0.5 | 4.9 |
| 4 | sand | 0.3-07 | 2.9 |
| 5 | Coal | 1.0 -1.2 | 1.45 |
| 6 | Quartz sand | 0.6-07 | 2.65 |
| 7 | Silica sand | 0.2 -2.5 | 2.67 |
| 8 | Gravel filter | 3-40 | 2.65 |
Filtration processes can generally be classified as being either slow or rapid. Slow sand filters are the original form of filtration. The first was built in 1804 by John Gibbs of Pusley, Scotland to treat water for his bleachery, with the surplus treated water sold to the public
FIGURE 1,2, and 3 TYPES OF FILTRATION MEDIA
DESCRIPTION OF SAND FILTRATION PROCESS
Slow sand filtration relies on both physical and biological activities in controlling plant pathogens. In a slow sand filter, the filter bed is constructed of a medium with high surface area which can be colonized by suppressive micro-organisms. This fine media also present a physical barrier to the passage of spores of plant pathogens. Bacteria, such as representatives of the genus pseudomonas and trichoderma have been demonstrated as biological control agents effectively controlling plants pathogens in hydroponics systems. In a slow sand filtration, plant pathogens recirculation in the irrigation water are captured in the filter media and at slow rates of water filtration (100-200l/hr/m2 surface area of filter), are acted upon by the antagonistic micro organisms that colonized the filter bed.
The efficiency of slow sand filter depends on the particle size distribution of the sand, the ratio surface area of the filter to depth and the flow rate of water through the filter. The finest grade sand fractions and granulated rock wool have been shown to be most efficient in controlling diseases such as phytophthora, pythium and jusarium oxysporum, the most wide spreed nursery diseases.
EFFECTS OF SAND FILTRATION ON WATER
1. Excludes fish and removes leaves, sticks and other large debris
2. Breaks down colloidal stability. A just PH for optitnum coagulation .
3. mixes chemicals with raw water, containing fine particles that will not readily settle or filter out the water.
4. gather together fine, light particles to form large dumps (floc) to aid the sedimentation/flotation and filtration processes.
5. Sedmentatation settles out large suspended particles flotation floats out the particles with dissolved air.
6. Rapid gravity filtration filters or removes remaining suspended particles. Slows and filters or removes remaining suspended particles. Slow sand filtration also involves biological action
7. Kills/inactivates disease – causing organisms. Provides chlorine residual for distribution system where chlorine is used.
8. helps control corrosive properties of water
9. helps control denial caries in children and young adults.
10. Stores water prior to discharge to service reservoirs.
SAND FILTRATION SYSTEM
This is a system which beds of granular materials or sand, drained from underneath river beds are being used in a sand filter for water purification. The sand filter serves primarily to remove Total suspended solid (TSS), phosphorus and insoluble organics from the water.
Slow sand filter are the original form of filtration. the first one was built in 1804 by John Gibb of paisley Scotland to treat water for his bleachery, with surplus treated water sold to the public sand filters were first used in London to treat water from the River Thames. From about the 1930s, water treatment by coagulation and rapid gravity filtration or pressure filtration tended to replace slow sand filtration in new plants and, in some cases, slow sand filters were replaced by rapi gravity filters following introduction of a coagulation stage. The slow sand filtration process has come back into favour in recent years due to its superior ability, compared to rapid gravity filtration to remove pathogenic micro-organism such as Girardia Lamblia and cryptosporidium.
TYPICAL WATER TREATMENT PROCESSES
MAINTENANCE OF SAND FILTERS
Inspections of sand filters and other pretreatment system should be conducted every six months and after storm events as needed during the first year of operation, and annually thereafter it is performed annually as designated. Suggestion for maintenance includes.
a) The accumulated debris and silt on top of the sand filters should be removed when their depths exceeds 12.5cm. The silts should be scraped off during periods with steel rakes or other devices. Once sediment is removed, the design permeability of the filtration media can typically be restored by … the surface layer of the media. Finer sediments that have penetrated deeper into the filtration media can reduce the permeability to unacceptable levels necessitating replacement of some or all of the sand.
b) Rapid drawdown in the sand bed (greater than 300mm per hour) indicates short circuiting of the filter. Inspect the cleanouts on the underlain pipes for leakage.
c) Formation of rills and gullies on the surface of the filter indicates proper function of the inlet flow spreader, or poor sand compaction. Check the accumulation of debris on or in the flow spreader and refill rills and gullies with sand.
d) Frequent overflow into spillway or overflow structure or slow draw down are indicators of plugging problems. The filter should be emptied in 24hours following a storm event, depending on pond depth. If the hydraulic conductivity drops to 1-inch per hour corrective action is need like
(i) scaping the top layer of fine-grain sediment accumulation (ii) Removal of thatch
(iii) Aerating the filter space
(iv) Tilling the filter surface
(v) Replacing the top 4-inches of sand
(iv) Inspecting geo-textiles in favor of dogs
(v) Avoid driving heavy equipment on the filter to prevent compaction and rut formation.
METHODS OF FILTRATION
With reference to the rate of flow through gravity filter, the methods may be classified as;
1) Constant Rate Filtration with Fixed Head: Here, the flow through the filter is maintained at a constant rate. They are either influent controlled or effluent controlled. Pumps or weirs are used for influent control where as an effluent modulating value that can be operated manually or mechanically is used for effluent control.
2) Constant Rate Filtration with variable Head: Here, the flow through the filter is maintained at a constant rate pumps or weirs are used for influent control. When the head or effluent turbidity reaches a preset value, the filter is backwashed.
3) Declining Head Filtration with Fixed or Variable Head: Here, the rate of flow through filter is allowed to decline as the rate of head are either influent controlled or effluent controlled. In the influent controlled types, the filter effluent lines are connected to a common header. A fixed orifice is built into the effluent piping for each filter after washing will take an undue share of the flow. The filtered water header pressure may be regulated by a throttle valve which discharges to filtered water reservoir. Costly rate controllers are placed with units economical particularly in large water works involving batteries of filters for equal duration of filter runs, the total output per day from a declining rate filter is higher than that in the conventional filters. Usually, the depths of filter boxes for declining rate filter are more than those for the conventional ones. These would permit longer filter runs and consequently reduce water requirements.
COMPARING THE PHYSICAL, CHEMICAL AND BIOLOGICAL CHARACTERISTICS OF WATER SAMPLE COLLECTED AT KWA RIVER BEFORE FILTRATION (RAW WATER) AND AFTER FILTRATION (FILTERED WATER)
A water supply filtration scheme based on sand filtration technology was implemented for the people of Ikot Effang Mkpa community in Calabar municipality in Cross River State. the system is reliable, inexpensive, easy to operate and very effective in removing both suspended materials and bacteria with little chemical addition. This river is located at Ikot Effanga Mkpa in the northern areas of Calabar with longitude of 4057’N and 8019’E. This River is highly contaminated due to human activities that takes place in the area.
TABLE 2; PHYSICAL AND CHEMICAL PROPERTIES OF KWA RIVER BEFORE AND AFTER FILTRATION
| Mean RW | Mean FW | Maximum Permitted for Nigeria standard | Maximum Permitted for WHO Standard | Remark | |
| PH | 7.6mg/l | 6.6mg/l | 6.5-8.5mg/l | 6.5-8mg/l | |
| suspended Solids | 12.29mg/l | 9.7mg/l | Not provided | Not Provided | |
| Chloride | 12.1 mg/l | 17.1 mg/l | 250 mg/l | 250 mg/l | |
| Sodium(Na) Amonia | 30.5 mg/l | 25.5 mg/l | |||
| Nitrite | 0.32 mg/l | 25.5 mg/l | 200 mg/l | 200 mg/l | |
| Nitrate | 2.9mg/l | 3.04mg/l | 50 mg/l | 50 mg/l | |
| Dissolved 02 | 4.02 mg/l | 3.56 mg/l | 5 mg/l | 5 mg/l | |
CONCLUSION
From the result above, it is seen that the water became improved after the filtration process and also meets the Nigerian thinking water standard as well as the WHO standards of drinking water. Sand filters have proven effective in removing several common pollutants, from storm water runoff. Sand filters generally control storm water quality, providing very limited flow rate control.
Ideally, the effective size is usually just small enough to ensure a good quality outflow and prevent penetration of clogging matters to such depth that it cannot be removed by surface scrapping, which is 0.30mm. finally little chemical treatments are added to the water like disinfection/sterilization, Ph correction, and fluoridation as they be desired.
REFERENCES
British Effluent and water Association (1993) “Standard for the Specification Aproval and testing of Granular filtering material BEWA: P. 18.93.
City of Austin, Texas June 20, 1990 Memo from Leslie Tull, Water Quality management Section. Updated December 14, 1998
Galli, J., 1990. Peat Sand Filters: A proposed storm water management practice for urbanized areas. Metropolitan Washington council of governments.
Naval Facilities Engineering Services Center, 1997 “Sand Filter for Treating storm water runoff”. Joint service pollution prevention opportunity Handbook, version 1.1 internet site at [http://enviro.nfesc.navy.mil/p2library/10-1297.html]. accessed June 1999.
Skarer, E. 1991. Sand Filter Design for water Quality Treatment Delaware Department of Natural Resources and Environmental Control. Updated December, 1998.
Huisman, L. & Wood W. E (1974). Slow and Filtration, Geneva, World Health Organization.
Smethrust, George (1990) Basic Water Treatment. Second Edition, F. Thomas Telford Ltd., London
Peavey, Hpward, Rowe, Donald and Tchobanglons (1986) Environmental Engineering Mc Graw-Hill, New York.
Flanagan P. J., (1992) Parameters of Water Quality – Interpretation and Standards. Second Edition. Environmental Research Unit, Dublin.
We are thankful to Mr. Abaraogu, Udechukwu John and Dr. Enoh C. Ukpong for submitting this very useful paper on Sand Filtration In KWA Community Of Cross Rivers State to us. We are hopeful that this will be of help to all those who are looking to study more on sand filtration.
If you have a query, you can ask a question here.