Removal of Suspended Substances in Domestic Wastewater

Removal of Suspended Substances in Domestic Wastewater by Coagulation Using Slow Sand Filtration and Roughing Filtration

Water to be supplied for public use must be potable i.e., satisfactory for drinking purposes from the standpoint of its chemical, physical and biological characteristics. Drinking water should, preferably, be obtained from a source free from pollution. The raw water normally available from surface water sources is, however, not directly suitable for drinking purposes. The objective of water treatment is to produce safe and potable drinking water. Some of the common treatment processes used for water purification includes Plain sedimentation, Slow Sand filtration, Rapid Sand filtration with Coagulation-flocculation units as essential pretreatment units. Pressure filters and diatomaceous filters have been used though very rarely. Roughing filters are used, under certain circumstances, as pretreatment units for the conventional filters. This paper specifically deals with Removal of suspended substances in domestic wastewater by coagulation using slow sand filtration and roughing filtration.

The application of micro filtration (MF) and ultra filtration (UF) via slow sand and roughing filtration for drinking water purification became a standard during the past two decades. This micro filtration process which uses slow sand and roughing filtration will hence forth be referred to as MFP in this paper. The first full scale applications in this field were reported in 1988 (Amy, 2006). In the meantime MF and UF applications have developed into widely established methods of primary treatment, with a steady increase in installed production capacity in the recent decades (Furukawa, 2002) both in the European Union and the United States.

By replacing conventional treatment steps (coagulation, sedimentation, and rapid filtration) with microfiltration a more reliable, robust, effective, and cheaper treatment method is introduced (Mallevialle et al., 1996). Compared to traditional treatment methods further advantages are (a) stable process under varying feed water quality, (b) smaller footprint, and (c) highly automatic operation. Most full-scale treatment plants are designed with polymeric MF/UF membranes. On the technical lifetime basis for the purchase of membranes for an entire drinking water treatment plant, the employment of ceramic membranes compared to polymeric ones was not a competitive alternative due to higher cost. Just recently studies in pilot scale (Loi-Brugger et al., 2006; 2006a), (Lerch et al., 2005) (Lerch et al., 2006) suggested that the process of purifying coagulated surface water with monolithic ceramic microfiltration membranes in constant flux and dead end mode can be optimised to such an extend, that their employment is competitive to the application of polymeric hollow fibre membranes. Higher flux and less frequent cleaning of ceramic membranes, but also considering the longer membrane lifetime is the basis for this recent leap in productivity.

Body

Slow Sand Filtration

Due to their size, slow sand filtration systems are typically designed specifically for each site and application. Package slow sand treatment units are available but are not commonly used.

Filtration Rate

The primary design parameter for slow sand filtration is the filtration rate. Design filtration rates typically range from 0.05 gpm/ft2 to 0.1 gpm/ft2 although rates as high as 0.15 gpm/ft2 may be tolerated for short periods during filter scraping or ripening. Filtration rates can have a significant impact on filter run lengths. Lower filtration rates may provide longer filter runs. The appropriate filtration rate should be determined by pilot study on the raw water to be treated. Using the design filtration rate, the required filter area can be determined for the design flow rate.

Number of Filter Basins

Since slow sand filtration requires that a filter be off line for up to two weeks for scraping and filter ripening, more than one filter basin is typically necessary. State regulations require multiple filter units that provide redundant capacity when filters are out of service for backwash or maintenance. This requirement may be waived for non-community water systems providing engineering justification acceptable to the WSDOH. Each filter basin that can be operated independently is considered an individual filter unit.

The number of filter basins provided will depend on the difference between average and peak flows, the anticipated filter run time, and available storage within the water system. The most conservative system design criteria would be to use the maximum day demand as the design filtration rate with one filter basin out of service.

Basin Materials of Construction

Filter basins can be constructed using concrete or earthen berm construction. For very small systems (

Regardless of the construction material, the tank should be made as watertight as practical because filtered water is collected in the bottom of the tank. For concrete tanks, water-stop material should be used at all construction joints. Hydrostatic relief valves should not be used. For earthen berm construction, continuous geomembrane liners should be used. Integrity testing should be performed on all geomembrane liner seams to verify no leak paths are present. Care should be taken when installing underdrains and gravel materials on geomembrane liners so as not to damage the liner material. It is WSDOH's position that common wall construction not be used between basins containing filtered water and unfiltered water due to the potential for contamination.

Geomembrane lined earthen berms are typically less expensive to construct than concrete basins but they have a shorter design life. The design life of a geomembrane is typically not greater than 20 years. The design life for a concrete basin is typically 40-50 years. Geomembrane liners are also not as durable as concrete basins as they can be damaged by activities such as sand scraping and resanding. Additionally, geomembrane liners must meet the requirements of WAC 246-290-220 pertaining to materials used in public water systems.

In order to understand fully the operating principles of a slow sand filter, it is necessary to have a basic knowledge of the way in which the filter acts, both biologically and physically. The treatment process is totally natural and is simply dependent on the maintenance of the correct environment for the growth of certain 'good' micro-organisms on or near the surface of the sand filter. Soon after the start of the treatment process, a film of these biologically active microorganisms develops in the filter fabric and the top of the sand. This film breaks down the incoming disease-carrying organisms, converting them into water, carbon dioxide, and other harmless chemicals. At the same time a large amount of suspended matter (which causes the cloudy or 'dirty' look of the water) is retained in the fabric and sand by simple straining (Amy, 2006).

The continuous straining process will gradually block the pores in both fabric and sand, which allow the water to pass through. This is shown by a lowering of the water level in the head-loss indicator tube on the outside of the filter tank, while the water level above the sand remains the same. In order to maintain the same flow through the filter, it is necessary to open the outlet valve further. After a certain time (generally 3-12 weeks, depending on raw-water quality), the valve will be fully open and the filter will be so blocked that it is no longer possible to get enough clean water out of it. It is then necessary to clean the filter (Amy, 2006).

Roughing Filters

The roughing filters systems are generally constructed in T11 tanks and developed to ensure that the raw water moves up-wards, which significantly enhances their cleansing effectiveness by utilizing gravitational forces to backwash amassed suspended solids accrued within the filter. Productivity is additionally enhanced by putting media on the elevated floor having a void beneath it (Pacini et al., 2005).

The amount of roughing filters needed is determined by raw water standard and needed production capacity which ought to be assessed/calculated as well as suitable models drafted before manufacturing begins. For maximum overall performance roughing filters ought to be operated in a maximum surface-filling rate of 0.6 m3 / m2 / hour. (This particular throughput may be seen as being the standard velocity in which water moves from the filtration system.) This implies that each and every T11 tank ought to be operated to generate 3.2m3/hour. They could be run at reduced throughputs however there's small distinction in percentage elimination of suspended solids in between 0.3 and 0.6 m3 / m2 / hr. Nevertheless, the effectiveness drops off over 0.6 m 3/m2 / hour. Circulation meters haven't been supplied in this system as they're vulnerable to congestion with higher suspended solids filling. Circulation rates will have to be established physically (Pacini et al., 2005).

Roughing filters are frequently recommended to be constructed with an in-sequence range where every tank becomes a phase, utilizing steadily finer media in every tank. Unprocessed water standard will decide the number of phases, i.e. how many roughing filter tanks will probably be needed: the more phases utilized (generally not more than 3) the higher the cleansing impact on the unprocessed water. When the water is relatively clear, a single-stage filtration system, or a tank with 3-media levels, might suffice. At least one small-scale pilot research should be carried out in order to signify the very best style from the method. Such tests ought to consider seasonal adaptations in water standard (Pacini et al., 2005).

Roughing filters ought to try to create water that's less than 20 NTU if water will be passed via a sluggish sand filter or less than five NTU if it's to become disinfected with chlorine. Where multi-sized media are utilized, it's common to choose coarse, medium and fine sized media. These may be configured in 1 multi-layered roughing filter, or with 1 media size in every tank, providing a three-stage method (Pacini et al., 2005).

The larger sized suspended particles within the unprocessed water, and some of the better ones, are eliminated with the rough media layer. Since this level has the biggest amount of substance to adsorb, it's the thickest in the 3 tiers. In cases where poor unprocessed water quality demands the building of the three-stage (i.e. three-tank) method, then these tanks ought to be constructed in sequence, utilizing 1 media dimension in every tank. The rough media tank is positioned upstream (Lin et al., 2008).

A roughing filter constructed of 3 layers within 1 tank features an elimination proficiency of Eighty five Percent at 0.3m 3 / m2 / hour as well as Seventy five Percent at 0.6 m3 / m2 / hour. 3 roughing filters in sequence possess an elimination proficiency of 87-92% whenever powered at 0.3-0.6 m3 / m2 / hour (all for turbidity variety 30-500 NTU). However, it should be noted here that accurate treatment specifications ought to always be based upon performing pilot tests around the actual unprocessed water that need be treated (Lin et al., 2008).

The design involving the roughing filter may rely on the approach utilized to deal with water in opposition to microbiological contaminants prior to usage. For instance, 3 roughing filters in sequence will decrease suspended solids by Fifty to five NTU, an amount adequately reduced to enable chlorination, whilst just one multi-layer roughing filter will cut down water from eighty to twenty NTU, permitting treatment with slow sand filtration system (Lin et al., 2008).

A steady suspension having a huge volume of organics, e.g. algae or even colloidal matter as well as color, might be hard to handle with a roughing filtration system and can occasionally need the inclusion of coagulants (like aluminium sulphate) upstream from the filtration system (Lin et al., 2008).

Having evaluated production prerequisite and also completed the pilot research using the unprocessed water under consideration, the suitable media size(s) may be chosen, the amount of T11 tanks as well as roughing filter systems purchased along with a method layout strategy drafted. Work may then move forward towards design (Lin et al., 2008).

Roughing filters carry out perfectly in a throughput (velocity) of 0.6m/hr, i.e. equal to a manufacturing capability of 3.2m3 / hr for the T11 tank. Flow management via filters ought to either be with floating continuous head outlet within the unprocessed water tank or perhaps with the gate device in the tank inlet. (A flow meter isn't offered together with the system because it is vulnerable to obstructions where suspended solids filling are higher.) (Skouras et al., 2007)

The tank ought to be loaded bottom-up (because it will probably be operated) to be able to drive out stuck air. When its run for the first time, water has to be enabled to go to waste for approximately 30 minutes to assist the system in becoming steady. In the same way, once the filtering system has been idle for some time, there'll most likely be several (extreme) solids moving and this should also be allowed to go to waste. Nevertheless it's more suitable to operate roughing filters as a continual procedure as opposed to on a batch schedule, because they operate much more effectively this way, each in relation to manufacturing output as well as in relation to water quality. Roughing filters might take 30 days before bedding down totally. The roughing filter shouldn't be permitted to dry up during an ongoing operation unless of course it's cleansed ahead of time. Drying out is not likely to happen because the outlet is on top of the container, but when it actually does, one should cleanse the tank a couple of times using influent water to be able to wash the bedded solids (Skouras et al., 2007).

The regularity of roughing filter cleansing will probably be based on local conditions; however the systems are usually dependable regardless of whether higher or lower levels of buildup have happened inside the container. The regularity for cleaning procedures also has modest impact on general overall performance (Skouras et al., 2007).

Additionally, higher degrees of organic matter demand much more repeated cleaning. Cleansing frequency may have to be based on actual encounter, although care ought to be taken to not "overclean," because there will probably be a loss involving biological effectiveness of the filtration system. Cleansing is conducted by shutting the inlet device and after that opening all 4 outlet valves concurrently to attain optimum backwash speeds, i.e. The tank ought to be purged out as swiftly as is possible to scrub out as many of the settled solids as is possible. The particular layflat hose pipe connected with the washout valves ought to be diligently laid out so that it doesn't have exceedingly tight flexes or kinks inside it, since this may significantly slow down the rate at which the container may be purged (Skouras et al., 2007).

At the very least, the T11 tank ought to be in a position to empty in four minutes or perhaps in two minutes to attain the right washout speed (the cleansing velocity must be 30m/hr or preferably 60-90m/hr). This might not be attainable with only 4 washout valves in the tank's bottom, therefore consideration might have to be directed at utilizing a pump to include cleansed water into the topmost part of the tank whilst draining to preserve the head of water within the tank to assure adequate scour is attained. There ought to be a comprehensive manual cleansing just about every Five years. The pinnacle loss inside a roughing filtration system ought to be within the zone of 200-300mm. If this has been surpassed significantly this can be additionally an indication that this particular filtration system ought to be cleansed. Calculating just how much dirt remains within the backflush water is really a helpful strategy for tracking the performance of the backwash program (Skouras et al., 2007).

Assessment and Review

In conventional water treatment coagulation has the objective of turbidity and color removal, both attributed to colloidal particles of organic and inorganic origin, like clays, and micro organisms. For coagulation and flocculation followed by sedimentation and filtration it is essential to create flocs with properties supporting settling in reasonable time and attachment on granular media filters. In microfiltration (MF) suspended solids and bacteria are primarily removed by the slow sand or roughing filtration process (MFP). Coagulation prior low pressure membrane filtration follows different objectives than in conventional processes.