12.3 Slow Sand Filtration (Subclass 4)
Key Takeaways
- DEP defines Subclassification 4 as particulate removal by physical and biological mechanisms during passage of raw water through a sand bed at low velocity, generally less than 0.4 meters per hour.
- Removal depends on the schmutzdecke, a biologically active layer at the sand surface, so slow sand filters are cleaned by scraping the top one to two inches rather than by backwashing.
- After scraping, the filter must be ripened by filtering to waste, commonly for hours to several days, until the biological layer re-establishes and effluent turbidity and coliform results are acceptable.
- Bed depth is reduced by roughly one inch per scraping, and when sand depth falls to about 18 to 20 inches the bed must be resanded, normally by trenching and returning the original top sand above new sand.
- Slow sand filtration requires low-turbidity source water without coagulation, is intolerant of algae blooms and of any chemical that kills the biological layer, and cannot be run at conventional loading rates.
A Biological Filter, Not a Mechanical One
Slow sand filtration is the oldest municipal drinking water treatment process still in regular use, and Pennsylvania certifies it as its own subclassification because it is operated in a fundamentally different way from every other filter. DEP defines Subclassification 4 as a process for substantial particulate removal by physical and biological mechanisms during passage of raw water through a bed of sand at low velocity, generally less than 0.4 meters per hour.
That velocity is the first thing to internalize. Converted to the units Pennsylvania operators use, 0.4 meters per hour is about 0.16 gallons per minute per square foot, and DEP teaches that loading rates of 0.04 to 0.08 gallons per minute per square foot are common in practice. Compare that with 2 to 6 gallons per minute per square foot for a rapid sand or dual-media filter: a slow sand filter needs on the order of 20 to 100 times the surface area for the same production, which is why the technology is used only where land is available and demand is modest.
DEP also has a mechanical distinction worth memorizing: a slow sand filter's components are the filter box, underdrain and media only. It has no surface agitator and no backwash system, because the underdrain collects filtrate but never distributes wash water.
Source Water Screening Criteria
DEP publishes explicit numbers for deciding whether a source can be run on slow sand:
- Turbidity less than 10 NTU, with particular caution when the turbidity comes from fine colloidal clays
- Total coliform not exceeding 800 colony forming units per 100 mL
- Raw color no greater than 5 units
- Low, stable algae, since blooms blind the surface quickly
And regardless of how good those numbers look, a pilot evaluation should be conducted before committing to slow sand filtration.
The Schmutzdecke
Within days to weeks of startup, a biologically active mat develops at the sand surface. This schmutzdecke (German for dirt cover) consists of algae, bacteria, protozoa and captured particulate matter. It performs most of the treatment:
- Physical straining of particles, cysts and oocysts
- Predation on bacteria by protozoa within the layer
- Biological oxidation of dissolved organic matter as water passes through the upper sand
Because treatment is biological, the operator's job is to keep the layer alive. Anything that kills it, most commonly prechlorination, destroys the filter's performance. Slow sand filters are operated without coagulant and without a pre-oxidant applied to the filter influent.
Typical Design and Operating Parameters
| Parameter | Typical value |
|---|---|
| Loading rate | 0.04 to 0.08 gpm/sq ft (less than 0.4 m/h) |
| Sand effective size | 0.15 to 0.35 mm |
| Uniformity coefficient | Less than about 2.5 |
| Initial bed depth | 30 to 40 inches of sand over graded gravel |
| Minimum bed depth before resanding | About 18 to 20 inches |
| Supernatant water depth | 3 to 5 feet above the sand |
| Terminal head loss | About 4 to 6 feet |
| Run length between scrapings | Weeks to several months |
Two Operating Modes
A slow sand filter is run in one of two modes: influent flow control, where a valve or weir on the inlet sets the rate and the supernatant depth rises as the bed plugs, and effluent flow control, where a valve on the outlet holds the rate constant as head loss increases. Some plants use the influent overflow line to maximize available head. The outlet chamber matters in both modes because it prevents the filter from being accidentally dewatered and going air bound, and the finished water holding tank downstream both supplies disinfection contact time for CT credit and buffers the filter's steady production against varying system demand.
Cleaning: Scraping, Not Backwashing
DEP describes three cleaning methods:
- Scraping — the most common and most effective. Drain to just below the top of the bed, then use an asphalt rake or shovel to pull a thin layer, roughly an inch, into parallel windrows, shovel the windrows into buckets, wheelbarrows or carts, and haul them out. Large filters are scraped mechanically with tractor-mounted scrapers. Afterward the bed is smoothed with the back of the rake or a section of wire mesh. Raising the water level before scraping lets loose debris float out the influent overflow so there is less to shovel.
- Raking — loosening the schmutzdecke surface with an ordinary garden rake to reopen surface area and extend the run without removing sand. It saves labor, but it pushes debris deeper into the bed, so the next scraping has to remove more media.
- Wet harrowing — agitating the surface under water so that suspended material is carried off in the overflow.
The removed sand is either discarded or washed and stockpiled for later resanding.
After scraping, the filter must be ripened:
- Backfill slowly from below, so trapped air is displaced rather than driven into the bed, and the media is not disturbed. A truly upset filter can only be cured by removing the sand and gravel and rebuilding it, which is expensive and labor intensive.
- Filter to waste while the biological layer re-establishes.
- Ripening commonly takes hours to several days depending on temperature and how deeply the bed was disturbed. Cold water lengthens it substantially.
- Return to service only after turbidity and bacteriological results confirm performance.
Because ripening removes a filter from production for an extended period, a slow sand plant is designed with multiple beds so that one can be down while others carry the load. Scheduling scrapings for mild weather and never scraping two beds at once are standard practice.
Resanding
Each scraping removes roughly an inch of sand. After enough cycles the bed approaches its minimum depth and must be resanded. The preferred method is throwing over or trenching: the top layer of biologically mature sand is set aside, new sand is placed beneath it, and the mature sand is returned to the surface. This shortens the ripening period dramatically compared with placing all new sand on top.
Where Slow Sand Fits, and Where It Does Not
| Suitable | Unsuitable |
|---|---|
| Small systems with a consistently clean source and available land | Any source with sustained turbidity above roughly 10 NTU |
| Systems seeking minimal chemical use and simple operation | Sources subject to heavy algae blooms, which blind the surface in days |
| Sources with low color and low total organic carbon | Sources with high color or organics, since slow sand removes little dissolved material |
| Operators available for periodic manual labor | Systems that cannot tolerate multi-day ripening outages |
Slow sand filtration provides strong Giardia and Cryptosporidium removal when properly ripened and operated, which is why it retains a place in Pennsylvania at small surface water systems, but it earns that credit only through disciplined scrape, ripen and return-to-service practice.
A slow sand filter is scraped and returned to service the same afternoon to meet demand. Finished water shows elevated turbidity and a coliform positive two days later. What was the operating error?
Which practice would most seriously impair the performance of a slow sand filter?
A slow sand bed originally 36 inches deep has been scraped repeatedly and now measures 19 inches. What action is required and what is the preferred method?