Wastewater Collection System Comparison

Onsite Facilities
Service Lines
Main Lines
Pumps
Treatment Facilities
Operation & Maintenance

Onsite Facilities

Effluent Sewer System	Conventional Gravity System	Grinder Sewer System
Septic Tanks: perform primary treatment - BODs: biochemical oxygen demand … removals of greater than 65 percent - TSS: total suspended solids … removals of greater than 70 percent are easily accomplished - Fat, oil and grease: removal of 90%± 4,000 litre tank per residence (commonly used where bedrooms are 3 or fewer) 24 hour emergency storage 24 hour hydraulic retention time for solids segregation 12 year pump out frequency Caustic chemicals and other system abuses are limited to individual tank	No tanks, therefore, no onsite treatment at each home Grit removal, comminution and primary treatment required at the treatment site. Lift stations are typical with every six to eight feet of fall. Caustic chemicals and other abuses passed into the main collection and treatment system. The adverse effects are passed down stream without simple means of detection for source identification.	Provide no treatment nor grease and oil removal Have little or no emergency storage or hydraulic retention time Requires service personnel to respond immediately to alarms Caustic chemicals and system abuses can be passed into the collection system with no dilution or primary treatment.

Service Lines

Effluent Sewer System	Conventional Gravity System	Grinder Sewer System
25mm diameter for STEP 32mm diameter for STEG and no minimum slope required	Minimum 100mm diameter Minimum slope required.	32mm diameter—no minimum slope required.

Main Lines

Effluent Sewer System	Conventional Gravity System	Grinder Sewer System
Shallow installations Follow contour of land: do not need to follow strict vertical or horizontal alignment. May be dug with trencher or backhoe Do not require minimum velocities Do not require periodic pigging - minimal fats Use low cost air release assemblies Designed using a "C" value of 150 Designs do not need to be oversized to accommodate hydraulic capacity for infiltration and inflow Because of tight line construction, smaller line sizes, shallower depths, etc., infiltration and inflows can be 99+ percent eliminated. Extremely low operation and maintenance cost. Odour control measures and abatement at open discharge to treatment or gravity sewer connections is much easier because of the low concentrations of solids and grease and oils in the effluent.	Typically 200mm diameter and larger Must be laid to grade Horizontal and vertical alignment is critical Large excavator usually required Designs require minimum line sizing and treatment capacity to accommodate the hydraulic impact of infiltration and inflow Require periodic hydro-flushing to remove solids deposited in main lines. Most gravity sewer districts own one or more flusher trucks (3 man crews), or contract with a cleaning service or neighbouring districts. Odour control is more prevalent and difficult due to high organic solids and grease and oil concentrations at the ends of pressure mains … as well as the decomposition of organic solids deposited in gravity segments.	Handle all liquids and solids in a macerated slurry Follow contour of land May be dug with trencher or backhoe Require minimum scouring velocities ( 2 fps±) Piping size is critical: if too small - excessive head loss; if too large - frequent pigging Frequent pigging is required due to high concentrations of grease and oils that adhere to the pipe walls Use expensive air release valves Designed using a "C" value of 120 Odour control measures at open discharge to treatment or gravity sewer connections is much more prevalent and difficult due to high organic solids and much higher concentrations of grease and oils.

Pumps

Effluent Sewer System	Conventional Gravity System	Grinder Sewer System
Constructed of non-corroding stainless steel and thermoplastic Lightweight-under 30 lbs. Long life- 20-30 years High head capacities: 1/2 Hp, 240V – 70m± 1 Hp, 240V – 120m± Small motors - 1/2 Hp, 240V with low amperage draw most common Easy to service and rebuild Low replacement cost. Complete servicing of a liquid end generally takes less than 30 minutes. Onsite pump replacement; generally require less than 10 - 15 minutes.	Lift stations require large, high flow, high horsepower, 3 inch ± solids handling pumps Require immediate response to pump failure Pumps are heavy and require machinery to remove and replace Pumps are expensive and difficult to maintain and replace. Repairs may take days or weeks Redundant pumps are required to accommodate emergencies. Pumps are over-designed to accommodate peak flows plus infiltration/inflow; therefore, power demands are unnecessarily high for normal usage. The use of small booster pumps to handle average flows is common practice	Corrode over time, even with protective coatings Heavyweight, over 100 lbs. Life- 5-10 years Lower head capacity: 1 1/2 Hp – 45m± Higher horsepower: 1 1/2 Hp - 2 HP most common Require 230V power Difficult to service, maintain, and rebuild Typically cost over $1000 to replace. Onsite pump replacement will generally require an hour or more and require two operators or special lifting equipment.

Treatment Facilities

Effluent Sewer System	Conventional Gravity System	Grinder Sewer System
Superior effluent quality: BOD=130mg/L TSS=30 mg/L FOG = 15 mg/L ± Easier to treat with either aerobic or anaerobic processes. Tight collection system allows for smaller line sizes and treatment facilities Suitable for sub-surface disposal Lends well to low maintenance treatment such as sand filters and sub-surface disposal Permit and testing requirements are considerably less restrictive and cost burdensome	Effluent quality of BOD = 200 mg/L TSS = 200 mg/L Infiltration and Inflow requires larger treatment facilities Unsuitable for use with sand filters and sub-surface disposal without substantial pretreatment Require surge capacity to ensure periodic peak flows don't degrade the treatment process.	Poor effluent quality, a macerated slurry of BOD = 300-450 mg/L TSS = 300-450 mg/L High fats, oil and grease concentrations (50 – 200 mg/L) Requires larger, more costly treatment facilities Unsuitable for use with sand filters and sub-surface disposal without substantial pretreatment

Operation & Maintenance

Effluent Sewer System	Conventional Gravity System	Grinder Sewer System
Long life pumps Large emergency storage capacity, maintenance personnel can respond when convenient Minimise electrical demands Minimal operation and maintenance costs and personnel training. Because of process configuration, high performance levels of treatment, long-term and consistent performance reliability, simple fail safe treatment mechanism … regulatory protocol is much less restrictive and cumbersome.	Require periodic flushing Larger treatment facilities require more maintenance … grit and solids removal primary segregation and clarification, secondary and final treatment processes, disinfection, sludge handling, etc. Greater continual power consumption due to aeration demands High level of operator training and certification required Much greater level of testing and regulatory requirement protocol to comply too.	Require periodic pigging due to high grease levels in the sewer lines Shorter lived pumps Limited storage capacity, maintenance personnel must respond immediately Higher onsite electrical demands Grinder mechanism should be rotated frequently (at a minimum every other year) and change out every other rotation Higher treatment power consumption due to aeration demands. Requires special tooling to handle heavyweight pumps Cost of rewinding pump motors typically exceeds cost of a new turbine pump High level of operator training and certification required Much greater level of testing and regulatory requirement protocol to comply to.