N.M. Admin. Code § 20.7.3.807

Current through Register Vol. 35, No. 21, November 5, 2024

Section 20.7.3.807 - LOW PRESSURE DISPOSAL SYSTEMS

A. Low pressure dosed (LPD) disposal systems are used to achieve uniform distribution of wastewater throughout the entire disposal system. Effluent is pumped under low pressure through solid pipe into perforated lateral lines installed within a disposal system.

(1) Low pressure dosed disposal systems may be used with any on-site liquid waste system including conventional treatment systems, gray water systems and advanced treatment systems.

(2) Low pressure dosed disposal systems may be used with any disposal system including trenches, beds, mounds, gravelless systems and evapotranspiration systems.

(3) Lift stations are not classified as low pressure dosed disposal systems.

(4) Low pressure dosed disposal systems may use a timer to equalize the flow over a 24-hour period. LPD disposal systems may also be designed to rotate between separate disposal areas by using rotator valves.

(5) All pumps shall be rated by the manufacturer for pumping sewage or effluent.

(6) A single pump may be used for design flows equal to or less than 1,000 gpd. Dual alternating pumps are required for design flows over 1,000 gpd.

(7) Design of the system shall include:

(a) design flow;

(b) except for mound systems, soil absorption area sized according to the effluent loading rates found in 20.7.3.703 NMAC;

(c) total length of header and lateral pipes;

(d) diameter of perforated lateral lines used;

(e) size and spacing of holes or emitters; and

(f) pump performance sizing with allowances for head and friction losses at rated flows in gallons per minute.

(8) A ball valve shall be located vertically at the end of each lateral line for inspection and flushing except for proprietary drip irrigation systems.

B. A low pressure pipe (LPP) disposal system is a pressurized distribution system placed in shallow, narrow trenches. The effluent discharged to a LPP system must meet, at a minimum, primary treatment standards.

(1) The low pressure pipe system shall be sized as follows.

(a) The required absorption area shall be sized in accordance with Subsection H of 20.7.3.703 NMAC.

(b) A sizing credit of five square feet per linear foot of lateral pipe shall be applied to the total required absorption area.

(c) Each individual lateral shall not exceed 75 feet in length from the feed point unless the design is such that the discharge rate between any two points in the system does not exceed 10%.

(2) Design for LPP systems shall conform to the following.

(a) Trenches shall be 12 inches to 18 inches wide and 12 inches deep.

(b) When aggregate is used, the lateral pipe shall be embedded at or above the center of the column of aggregate.

(c) The aggregate shall be covered with geotextile material to prevent soil intrusion.

(d) If a proprietary drainfield product other than aggregate is used, the distribution pipe shall be placed so as to prevent soil intrusion into the pipe.

(e) A minimum of four inches and a maximum of 18 inches of soil cover over the trench is required.

(f) Lateral lines shall be placed parallel to the natural contours of the site.

(g) Provisions shall be made for the prevention of siphoning back to the pump tank on upgrade systems and the prevention of draining of the tank on downgrade or flat systems.

(h) All requirements for conventional disposal systems shall be met, including but not limited to, setback and clearance requirements, lot size, design flow calculations, septic tank sizing, prohibitions, wastewater characteristics and advanced treatment requirements.

(i) Runoff shall be diverted away from the system to avoid oversaturation, where possible.

(j) A vegetative cover shall be maintained over the disposal area.

(3) Materials and equipment for LPP systems shall conform to the following.

(a) All treatment units and pump tanks shall meet the structural requirements of 20.7.3.501 NMAC.

(b) The pump tank shall be a single compartment with a 500 gallon minimum useful volume and allowance to be made for tank volume between the pump intake and tank floor. For septic tank effluent, a separate pump tank, in addition to the septic tank, is required.

(c) Effluent type pumps are required on all systems.

(d) A system design shall demonstrate that the system comes to the design pressure during every pumping cycle.

(e) An alternating valve or solenoid valve system is required to feed separate laterals with elevation differences resulting in 23 feet (10 psi) or greater head differentials. Manual or automatic flushing valves with turn-ups are required on distal ends of all laterals.

(f) In areas of freezing conditions, provisions for the draining of the headers must be made, such as vacuum breakers or vent holes at the system high points.

(g) Pipe shall be rated at 160 psi minimum, ASTM compression drainpipe, schedule 40 or better.

(h) The manifold pipe shall be sized appropriately for system size and configuration. The lateral pipe shall be one inch to two inches in diameter.

(i) The orifice size shall be 5/32 inch to 1/4 inch for septic effluent and 1/8 inch to 1/4 inch for secondary and tertiary treated effluent.

(j) The lateral pipe shall be installed with orifices facing upward.

(4) A maintenance contract shall be required on all LPP systems. Maintenance is to include pump inspection and cleaning, float operation (if applicable), lateral flushing annually at a minimum and septic tank and pump tank pumping as needed.

C. Designs that do not conform to the design parameters specified in Subsections A and B above must be accompanied by documentation justifying the design submitted, including proprietary software input and output reports, and will be considered on a case-by-case basis.

N.M. Admin. Code § 20.7.3.807

20.7.3.807 NMAC - Rp, 20.7.3.808 NMAC, 9/1/13; A, 9/1/13