Gravity sewer systems designed to permit on the first submittal, operate reliably, and transition to public ownership without complications. That's our standard.
Sanitary sewer collection systems are among the most critical infrastructure components of any land development project. Unlike water distribution or stormwater systems, sewer design failures have significant consequences: improper grades lead to line plugging and overflow, undersized pipes cause backups in buildings, and aesthetic or operational issues at transition points with municipal systems create liability that can follow a developer for years.
Hagen Engineering's approach to sewer design prioritizes three outcomes: first-pass permit approval, long-term operational reliability, and seamless municipal acceptance. We design systems that meet or exceed local code requirements and are built to be maintained.
Properly sizing a sewer system begins with accurate flow projections. We don't use generic flow numbers — we evaluate your specific project based on its land use, occupancy density, water service plan, and local water consumption patterns. We analyze peak hour flows, average daily flows, and future growth scenarios to right-size the collection system without overbuilding unnecessarily.
For commercial projects, we coordinate directly with tenants or tenant representatives to understand actual water usage and demand patterns. For multifamily developments, we account for expected occupancy, fixture counts, and fixture types. For mixed-use projects with retail, office, and residential components, we develop composite flow estimates that reflect the true operational profile.
Once flows are established, we conduct detailed capacity analysis on all proposed sewer lines. We calculate slopes, pipe sizes, and velocities to ensure design flows are conveyed without exceeding allowable velocities (typically 2-10 feet per second depending on pipe material and local standards) and maintaining minimum slopes to prevent sediment accumulation and line degradation.
Well-designed gravity sewer systems follow fundamental engineering principles that ensure decades of trouble-free operation. Every section of the collection system must slope continuously downhill to the municipality connection point. We evaluate site topography and existing infrastructure carefully to establish feasible grades that maintain proper flow velocities while minimizing excavation depth and cost.
Minimum pipe slopes vary by pipe diameter and material, but typically range from 0.4% to 1.0% for most collection systems. We never design flat or slightly uphill sections — even minor elevation changes can cause problems over time. When site constraints make steep grades necessary, we incorporate energy dissipation measures at manhole drops to prevent scour and erosion.
Pipe material selection depends on site conditions, local utility standards, and project requirements. Clay, concrete, PVC, and HDPE pipes are all viable options depending on groundwater conditions, chemical exposure, construction methodology, and municipal acceptance. We work closely with the receiving municipality to confirm acceptable materials before finalizing designs.
Maintenance access points are critical to long-term sewer system operability. We design manhole spacing, sizing, and locations based on line changes in grade, direction, or diameter — and also based on local municipality standards, which vary significantly across jurisdictions. Typical municipalities require access every 300-500 feet on straight runs, but spacing requirements may be tighter or more generous depending on local policy.
Manhole inverts (the internal bottom elevation where pipes connect) must be established with precision. We design details that ensure proper flow transition inside the manhole, minimize energy loss, and prevent scour or turbulence that could damage the structure or create operational problems.
We also specify whether each cleanout will be a standard manhole (with a 4-foot diameter or larger opening for human entry and maintenance equipment), a sanitary cleanout (a smaller 4-6 inch opening for mechanical cleaning access), or a terminal cleanout at the end of short branches. Each type serves a specific purpose and has different construction requirements.
When site topography or physical constraints prevent gravity sewer lines from operating underground at feasible depths, aerial crossings become necessary. These structures — typically steel pipe or concrete box culverts elevated above grade — require specialized design consideration including thrust block analysis, support pier design, and detailing that prevents sediment accumulation inside the pipe.
Aerial crossings are often controversial with municipalities because they have aesthetic impact and can create perception issues. We design these systems to minimize visual intrusion, ensure proper drainage and cleanout access, and provide compelling engineering justification to reviewing agencies. Where feasible, we explore alternatives such as deeper trenching or rerouting to eliminate aerial sections entirely.
Every municipality across the Southeast enforces its own sewer design standards. Some are clearly documented; others evolve through precedent and institutional knowledge. We maintain strong working relationships with permitting and utility engineers throughout our service territory and understand the specific expectations of each reviewing jurisdiction.
During the pre-application and design phases, we coordinate directly with the receiving utility to confirm design parameters, pipe materials, sizing calculations, and specification details. This proactive engagement prevents costly redesigns and expedites permit review. Many municipalities have specific details they require, spacing preferences for cleanouts, particular pipe materials they prefer or exclude, and performance standards for capacity analysis that differ from state guidelines.
We prepare detailed sewer reports documenting all flow calculations, capacity analysis, pipe sizing rationale, and compliance with local standards. These reports accompany permit applications and provide reviewing engineers with the justification for every design decision, significantly reducing review time and comment cycles.
Sewer system design cannot occur in isolation. Grades, pipe routing, and manhole locations must coordinate with site grading, stormwater systems, water distribution lines, and utility routing. We work within the overall site design framework to locate sewer infrastructure in areas that don't conflict with parking, building foundations, tree preservation, or stormwater management features.
We also design all connections between the site system and the municipal connection point. This includes the transition structure where site lines meet the municipal system, any required grease traps or treatment for food service establishments, and pretreatment systems if required by the municipality.
Excellent sewer design is constructible sewer design. We establish pipe slopes, manhole spacing, and construction sequencing that can realistically be built without excessive site modification or cost overruns. We consider equipment access, trench depth, dewatering requirements, and existing utility conflicts during the design phase so the contractor can execute the project efficiently in the field.
We also prepare detailed specifications and notes on construction documents that guide the contractor through proper installation, testing, and inspection procedures. Sewer lines are typically inspected via closed-circuit television (CCTV) before municipal acceptance, and our specifications ensure the work is done to a standard that passes inspection without remedial cleaning or repairs.
Hagen Engineering's team combines deep technical expertise in hydraulic and structural design with practical field experience. We design sewer systems that work — systems that permit quickly, perform reliably, and integrate smoothly into public utility ownership. If you're developing in the Southeast and need a trusted engineering partner for sewer collection system design, we'd welcome the opportunity to discuss your project.
Tell us about your project and we'll walk you through the process. Our engineers are ready to help you design a system that permits smoothly and performs reliably.