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9.16 CHAPTER NINE

FIGURE 9.6 Harrow tractor.

• Access ramp for harrowing equipment to enter the filter
• Harrowed water influent distribution system
• Harrowed wastewater collection system
• Holding lagoon for the harrowed wastewater
The filter box must be designed with enough headroom to permit tractor operation,
and support columns should be minimized to improve maneuverability. Figure 9.6 shows
a harrow tractor operating in a filter basin. Influent and waste headers can consist of ei-
ther perforated piping or channels with adjustable weirs.
Wastewater lagoons should be designed to carry one to two harrowings with the abil-
ity to allow time for settling and to discharge the decanted water in stages. Because waste-
water has generally not received previous chemical pretreatment, discharge requirements
are usually less rigorous.
The process is now currently used at several operating plants in the United States to
improve plant performance and reduce cleaning times. Designers of larger-capacity plants
should consider using this cleaning process.

DIATOMACEOUS EARTH FILTRATION

Diatomaceous earth (DE) filtration has been used effectively for drinking water treatment
since 1942 when it was adopted as a standard method for the U.S. Army. The DE filter
was selected because of its portability and effectiveness in removing Entamoeba histolytica
cysts (Black and Spaulding, 1944). These cysts are pervasive in some parts of the world
and are difficult to control with disinfectants alone. The capability of DE filtration to ef-
fectively remove particulates applies equally well to the latter concerns of Cryptosporid-
ium and Giardia cysts where cyst removals of approximately 6 logs have been achieved
(Ongerth and Hutton, 1997). DE filters are commonly called precoat filters because of the