The undermentioned treatment plants shall comply with the following requirements—
1. SEPTIC TANKS
(a) Except with the approval of the Public Health Board it shall be not less than 50 feet from any wall, spring or stream of water used or likely to be used by man for drinking or domestic purposes or for the manufacture or preparation of articles of food or drink for human consumption or for the cleaning of vessels with a view to the preparation for sale of articles and otherwise in such a position as not to render any such water liable to pollution.
(b) The effluent from every septic tank shall discharge through a drain not less than 6 feet in length into an approved constructed effluent disposal system which shall be not less than 50 feet from any well, spring or stream of water as described in paragraph (a) above.
(c) Shall have a minimum retention time of 24 hours and effective capacity from holding sludge for a minimum of 2–3 years based on the number of persons discharging to the tank on 1 DMF per person.
(d) The minimum size of a septic tank should be 3,500 litres or 700 gallons.
(e) Minimum water depth 1.5 meters (5 feet) to maximum 1.8 meters (6 feet).
(f) Shall be provided with adequate ventilation.
(g) The invert elevation of the inlet shall be one inch higher than the invert of the outlet pipe.
(h) The inlet and outlet of septic tanks shall be fitted dip pipes or dip plates to prevent the disturbances of the scum during normal operation.
(i) The dip pipes shall extend about 375 mm. (15 inches) below liquid level (below overflow level) but shall not extend more than 450 mm. (18 inches).
(j) The last section of the outfall drain to a septic tank shall not be laid on a steep gradient, maximum velocity 0.9 m/s or (3 ft./sec.).
(k) Septic tank construction shall be made with adequate strength to resist earth movement and to support the weight of the tank walls and contents and that the floor and walls be made water tight.
2. IMHOFF TANKS
Settling compartment detention:
2.5 to 3.0 hr. preceding bio-filters
1.0 to 1.5 hr. preceding activated sludge.
Sludge compartment capacity per person:
0.07 to 0.10 m3 for primary sludge
0.085 to 0.11 m3 for primary plus humus sludge.
3. SEWAGE SETTLING TANK
(a) Sewage settling tank shall not be used for population below 100 and shall only be used for larger population and arrangements must be made for disludging and disposal of the sludge at a minimum interval of once a week.
(b) Size of settling tank shall be 5 times the width and a minimum capacity of 12 hours Dry Weather Flow (DWF).
(c) Maximum horizontal rate of flow 12 to 15 meters per hour or 40 feet to 50 feet/hour at DWF minimum of 2 tanks in parallel.
(d) Sludge pipe work shall be at a minimum diameter of 150 mm. or 6 inches.
4. SEWAGE BIOLOGICAL FILTERS
(a) Filters beds may be circular or rectangular in plan. Rectangular filters may be permitted up to a population of 50 persons.
(b) Sprinklers for biological filters (small treatment works) (300–2,000 population) shall be provided with no jets or small opening. Shall distribute effluent evenly from maximum capacity down to a mere trickle.
(c) For population over 2,000 persons sprinklers shall be of the rotating type and approved by the Public Health Board.
(d) Distributer for population over 50 persons and not more than 2,000 shall be of the rotating type with water wheels weir box and tipping trough or alternating siphon arrangement.
(e) Maximum dosage rate of BOD loading in terms of cubic meters or effluent per cubic meter of medium per day shall be 0.6 cubic meter per cubic meter/day or 0.75 cubic meter of medium per head of population for filter serving up to 10 persons for larger works BOD loading 0.06–.09 kg. BOD per cubic meter/day will be accepted. 0.08 kg. BOD per m2 rotating disc surface area.
(f) Arrangements shall be made to dose filter frequently and continuously and in any case not less than 15 min. interval.
(g) Re-circulation of effluent shall be provided but may be allowed omission only with the permission of the Board.
(h) Depth of filter beds shall be 1.8 meters, 6 feet to 2.0 meter, 6 feet 8 inches depth of filter medium and all filtering medium must be approved by the public health officer.
(i) Filter media must be approved by Public Health Department before placed into use.
5. PACKAGE PLANTS
(a) Anaerobic-Aerobic:
18 hours anerobic contact retention
2 hours intermediate settling retention
6 to 8 hours aeration at air input of 63 m3/kg. BOD per day
5 hours final settling retention.
(b) Extended Aeration:
All plants shall be based on 3 DWF flow. Daily organic loading shall be 0.16 to 0.32 kg. BOD per cubic meter aeration tank capacity. Detention time shall be 24 to 48 hours in aeration tank and 4 to 7 hours in settling tank.
Air supplied shall be 130 to 300 cubic meter/kg. BOD loading.
Excess sludge withdrawal 9 to 10 litres per person/ week.
Excess sludge consolidation tank 6 to 8 weeks storage capacity with air bubble stirring and periodic decantation of supernatant.
(c) Contact — stabilisation:
Contact zone 2 hour retention giving one hour absorption period at 100 per cent recirculation or capacity of 10 m3 per 11 to 16 kg. BOD per day.
Settling 2 hours retention.
Re-aeration 5 hours retention with air supply of 63 m3/kg.
BOD per day.
Aerobic digester 85L/person storage capacity.
6. OXIDATION DITCH
BOD loading shall not exceed 210 mg/1 of ditch capacity per day with:
(a) A BOD/sludge loading factor near the lower end of 0.05 to 0.15 g. BOD per day per gram activated sludge.
(b) Oxygen capacity shall be twice the BOD load to be removed.
(c) Rotor shall maintain the required velocity in order that a velocity of 0.3 m/s (one foot per sec.) of sewage is maintained in ditch.
(d) Rotor shall be immersed a minimum of 50 mm. (2 inches) and maximum of 225 mm. (9 inches).
(e) A mixed liquor suspended solids shall be maintained at 4,000 to 4,500 milligrams per litre.
(f) The volume of ditch 120 to 150 cubic meter length of rotor for small unlined ditch but may exceed that for larger plants if a lining is used 300 litres per person.
(g) Organic loading shall be maximum of 160 kg. BOD per day per 1,000 cubic meter of ditch (i.e. 160 milligrams per litre).
(h) Final settling retention time shall not be less than 1.5 hours and a maximum upward velocity of 0.9 to 1.2 meter/hour.
(i) Depth of ditch shall not exceed 1.0 to 1.5 meter.
7. OXIDATION PONDS
Oxidation Ponds shall comply with the following:
(a) All oxidation ponds shall be a minimum distance of 532 meters or 1,750 feet from any housing area.
(b) There shall be strict control by the owner of vegetation and embankment erosion and mosquito breeding.
(c) Arrangement shall be made re circulation of effluent.
(d) No pond depth shall be less than 600 mm. (2 feet) and not more than 1,500 mm. (5 feet).
(e) The loading shall be a maximum of 170 kg/ha or 6,000 to 10,000 persons per hectare 250 and 275 kg BOD hectare/day.
(f) In special cases the Public Health Board may permit loading of 450 to 550 kg/hectare per day.
8. INTERMITTENT SAND FILTERS
Application rates of 0.10 to 0.15 m3/day per m2 of surface area for secondary treatment and 0.5 to 1.0 m3 day per m2 for tertiary treatment. A minimum of 2 beds provided to allow alternate dosing.