Overview of Automated Lagoon Dredging System: Design Features and Requirements

Liquid Waste Technology, LLC® (LWT) has pioneered the development of fully automated dredging systems (and radio remote controlled dredging) for the most efficient and safe cleanup of sludge lagoons. These systems are state-of-the-art, and LWT has many successful installations so that reliable operations are the norm.

Laptop computer screen showing automated dredging features with mouse controls

Laptop computer screen showing automated dredging features with mouse controls

The objectives for most automated dredging systems include:

  • Less human labor input, so costs less to operate
  • Safer because less human exposure, and fewer compliance issues
  • Steady solids output so more efficient sludge processing
  • Reduced polymer use, so significant savings (hard cost reductions in polymer use alone has frequently yielded one year payback or less on investment required)
  • Optimal sludge percentages reduce tank truck transport costs (minimize water weight)
  • Better record keeping capabilities

What follows is an overview of specific design features and requirements:

Dredge Slurry Pump:

LWT has different pump sizes and options to fit your needs. The solids handling impeller is capable of passing solids from 3 inches up to 8 inches. The pump casing, impeller and bearing housing is available in cast iron, high chrome, or stainless steel.

The hydraulic control system provides independent, variable speed capability within the pump’s design curve, with an electric speed control. The pump speed can be set and then maintained without further hands-on control, with the ability to adjust speed easily.

Dredge Controls

Shore Control Panel:

Shore Control Panel

Shore Control Panel

All controls necessary to start, stop, and control platform speed and operation are on a shore-mounted control panel. The control panel has electronic controls over the hydraulic valves which control the hydraulic functions of the dredge (i.e. pump, auger, traverse system and pump hoist winch) and the lateral move system.

The functions of the controls are as follows:

The Shore Control Panel should have the following functions, controls or displays:

  • On and off power switches with indicator lights
  • Slurry pump variable speed control
  • Raise/lower auger excavator
  • Auger excavator speed control speed rotary throttle, forward/reverse
  • Traverse speed control rotary throttle, forward/reverse
  • Select on board/shore control (if controlled on board)
  • Selector switch hand/automatic control, lateral move function
  • Indicator lift for lateral move operation
  • Automatic lateral move shift direction switch left/right
  • Manual lateral move start/stop
  • On board, the floating dredge has the following functions, displays or gauges:
  • Pressure on circuit for each hydraulic pump
  • Hydraulic oil level
  • Safety shutdown for hydrostatic drive and fluid level
  • Combination disconnect, starter and overload
    Programmable Logic Controller

    Programmable Logic Controller

Programmable Logic Controller:
The dredge can be controlled with mechanical switches or with a PLC (Allen Bradley or equal). With the inclusion of the PLC, the system can also be tied to a solids control loop program and/or flow control program. (See elsewhere on LWT web site for Solids Sense™ and Auto Sense™ for more information.)

All the controls are in an on-board control panel at a second shore mounted remote control panel located with the owner’s input. There are corresponding switches mounted on the shore control panel selecting operations between the panels.

The On-Board Control Panel and Radio Remote Hand Held Panel have the following functions, controls and displays:

  • On and off power switches with indicator lights
  • Slurry pump variable speed control
    Hand-held radio remote controller for LWT Runt model dredge

    Hand-held radio remote controller for LWT Runt model dredge

  • Elevation hoist raise/lower
  • Auger speed control, forward/reverse
  • Traverse speed control, forward/reverse
  • Selector switch hand/automatic control, lateral move function
  • Indicator light for lateral move operation
  • Automatic lateral move shift direction switch left/right
  • Manual lateral move start/stop
  • Slurry pressure gauge (optional)
  • Slurry density meter (optional)

The Remote Control Panel typically has the following functions, controls, and displays:

Hand-held Radio Remote Controller for 18 ft. model dredge

Hand-held Radio Remote Controller for 18 ft. model dredge

On and off power switches with indicator lights
Pumper on indicator light
Slurry pump with variable speed control
General pumper system warning light displays and alarm horn which activate due to any of the following conditions:
Failure due to low hydraulic oil level or temperature
Failure of lateral move system

Automated Rail Type Lateral Move System:

 

One of the keys to LWT’s automated lagoon pumping system is an automated lateral movement system designed for a completely automated movement of one sweep

Anchored Steel Rail System

Anchored Steel Rail System

over the entire lagoon when combined with a traverse system on the dredge. The lateral system consists of an anchored steel rail system on each of the short ends of the lagoon of sufficient strength to handle the tension forces developed by the pumping system. LWT can calculate the required strength. The system can be set to be operated in a manual or automated mode.

The system sequence is activated by the proximity switches mounted on the front and rear of the unit sensing the cable stops that are mounted on the traverse cable.
After a forward pass across the pond, the system automatically performs the following dredge functions:

(a) Reverse and increase travel speed, slow the pumping rate, travel to pass start

Proximity Switches

Proximity Switches

point.

(b) Then raise the auger head, side shift the selected direction and distance, lower the auger head, and start the next run. (Optionally, the unit could wait for a start command.)

The distance of lateral movement is controlled in the shore mounted control box by an adjustable run timer. Electronic sensors detect the completion of a sweep across the entire pond or the end of each rail.

Trolleys:

Trolleys

Trolleys

The lateral move system consists of steel rail and anchor post, wiring, cables, pulleys, turn-buckles, controls, drive motors, disconnects, reduction and triple sheave drive winch(es), limit sensors, trolleys, galvanized cable and miscellaneous accessories as required for a complete system. The owner provides incoming electric power to the switches and winch motors.

Each rail has a tightening mechanism (turn-buckle) attached to a pulley, an electric motor driven gear box reduced triple sheave winch and trolley. A loop of 3/8″ galvanized steel cable runs between the trolley, the pulley, and the winch. The traverse cable for the pumping unit attaches to this trolley and can be tensed with a grip hoist endless winch.

Triple Sheave Winch

Triple Sheave Winch

The on-shore control panel is housed in a NEMA Type 4 enclosure with controls and switches mounted on the face of the panel. Watertight sealed fittings are used where conduit enters the enclosures and any unused openings have watertight sealed covers. The main power supply and function control cable furnishes power from the disconnect on shore to the floating dredge.

The float ball system in the lagoon attaches to the discharge hose and power cable. The cable should be properly sized for a maximum of 3% voltage drop with color-coded conductor insulation. The conductors should be of stranded construction to maintain flexibility. The cable should be UL listed, type W as a minimum. All conductors should be assembled in a cable with fillers in the interstices. Cables should include strain reliefs and may be optionally equipped with quick couple pin connectors with matching receptacles on the appropriate shore boxes.

The dredge disconnect has double lugs to provide power to both the pump motor starter and the auxiliary equipment. The switch is a 3-pole manual switch rated for power amperes, 600 volts, 60

Cable Floats

Cable Floats

Hz, with quick-make and quick-break mechanism and housed in a NEMA enclosure.

The starter typically has 120 volt control. On board 120 volt power is supplied by a dry type transformer rated at a minimum of 3 KVA.

The following dredge system features chosen by the Sioux Falls, South Dakota project owner are optional but increase efficiency:

Automated Bottom Sensor:
The shroud is equipped with a 10″ (254 mm) roller system to prevent contact and hold the shroud 6″ (152 mm) off the bottom of the work area. The auger is equipped with a sensor unit that will automatically raise or “float” the head when contacting the bottom of the pond.

Gauge Wheels:
Adjusting guide wheels mounted to the shroud hold the shroud off pond bottom and protect the liner or bottom.

PLC:
The PLC is housed in the shore control panel and is programmed to control the automated traverse sequence and report the specific failure message to the shutdown read out. The PLC system will take control signals from shore, convert to 4-20 voltage or 4-20 ma signals, and communicate them to the dredge where they are converted back to DC to operate the electronic proportional valve.

Upon the occurrence of one of the following failures the beacon will activate, and the monitor will read out the fault:

  • Hydraulic oil level
  • Hydraulic oil temperature high and low
  • Hydraulic oil high pressure and low pressure
  • Lateral move motor failure

The same PLC is used with a solids control loop and a flow control loop.

Solids Sense™ Solids Control Loop:
This system uses a density meter and a PLC. The LWT density meter feeds density data to the PLC. The PLC is programmed to process the data.

If the density is lower than the adjustable target value, the PLC automatically commands changes in the lagoon pumpers’ operational functions, which are designed to shift the density results toward the target value. (For example, increase dredge forward movement to increase solids density.)

Flow Control Loop:
This system requires a magnetic induction flow meter and a PLC. The LWT flow meter feeds flow data to the PLC.

The PLC is programmed to process the data. If the flow is lower than the adjustable target value, the PLC will automatically command changes in the dredges operational functions, which are designed to shift the flow rate results toward the target value.